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«73 


THE  LIBRARY 

OF 

THE  UNIVERSITY 

OF  CALIFORNIA 


GIVEN  WITH  LOVE  TO  THE 

OPTOMETRY  LIBRARY 

BY 

MONROE  J.  HIRSCH,  O.D.,  Ph.D. 


THE  SHADOW  TEST 


In 

Ocular 
Diagnosis 


Without  prejudice  as  to  method,  mirror, 
distance  or  instrument 


BY 

GEO.  A.  ROGERS 

AUTHOR     OF    SKIASCOPY 

PRINCIPAL    CHICAGO    POST    GRADUATE 

OPTICAL    COLLEGE 

Professor  of  Optometry,  Northern  Illinois  College  of 

Ophthalmology  and  Otology 

President  Illinois  Optical  Society 


THE  OPTIC  PRESS 
CHICAGO 


u'ff'^y^ 


©PTOMETRY 


Copyright 


DP  To 

Prefatory 

During  the  past  five  years  the  author  has  been  asked 
many  times,  and  often  with  a  good  deal  of  insistence,  if  he 
had,  and  why  he  had  not,  embodied  his  methods  of  teaching 
optics  in  permanent  book  form.  Undoubtedly,  aside  from 
the  facts  involved  in  the  study  of  optics,  there  is  a  good 
deal  in  the  methods  of  imparting  instruction  to  the  initiate 
or  new  student  of  the  science. 

Wliile  there  can  scarcely  be  said  to  be  a  dearth  of  optical 
text  books  and  general  literature,  perhaps  the  manner  in 
which  the  subject  is  dealt  with  in  a  series  of  books,  of 
which  this  is  the  initial  volume,  will  be  of  assistance  in 
preserving  a  favorite  method  of  viewing  the  peculiar  phe- 
nomena of  optics  and  the  author's  explanation  of  their 
scientific  meaning.  This  is  the  best  excuse  we  can  offer  for 
perpetrating  upon  opticians  and  would  be  opticians  a  new 
series  of  optical  text  books. 

The  series  will  embrace  at  least  four  separate  volumes, 
none  of  them  large  enough  to  be  unwieldy,  but  strongly 
bound,  so  that  they  can  be  used,  in  a  pinch,  to  split  kind- 
ling wood  with,  or  be  thrown  out  of  a  sixth  story  window 
without  serious  damage,  as  we  feel  sure  some  of  their  read- 
ers will  be  tempted  to  do.  The  series  will  be  issued,  if 
they  are  ever  completed,  in  the  following  order: 

Vol.  I,  The  Shadow  Test,  etc. 

Vol.  II,  Ophthalmic  Lenses,  Mirrors  and  Lens-Mirrors. 

Vol.  Ill,  The  Ocular  Muscles  and  Muscle  Testing. 

Vol.  IV,  Practical  Optometry  (a  general  text  book). 

The  author  hopes  in  these  four  volumes  to  embody  all 
he  knows  about  optics  and  perhaps  considerable  more.  If 
he  happens  in  the  mean  time  to  think  up  anything  else 
it  may  possibly  appear  in  a  fifth  volume,  but  there  is  no 
present  intention  of  going  farther. 

Chicago,  May  15,  1905.  G.  A.  R. 


Illustrations. 

1.  Shadow  Testing-,  Plane  Mirror  at  One  Meter. 

2.  Conjugate  Foci,  and  Conjugate  Planes. 

3.  Mirror  and  Source  of  Light. 

4.  View  of  Patient,  through  Peep  Hole  of  Mirror. 

5.  Testing  Emmetropia,  Plane  Mirror  at  One  Meter. 

6.  The  Fundus  Reflex,  and  Motion  with  Plane  Mirror. 

7.  Testing  Hyperopia,  Concave  Mirror  at  53  Inches. 

8.  Testing  Astigmatism,  Plane  Mirror  at  one  Meter. 

9.  The  Reflex  in  Astigmatism,  Band  and  Straightedge. 

10.  Movements  of  Real  Light. 

11.  Peculiar  Reflex  Effects. 

12.  Shadow  Test  Devices. 

13.  The  Geneva  Retinoscope. 

14.  The  Cross  Retino  Skiameter. 

15.  The  Hardy  Stigmatometer. 

16.  Shadow  Test  Practice  with  Schematic  Eye. 


Introduction 

Since  optometry  as  a  practical  science  and  art  first  ap- 
peared upon  the  stage,  the  most  distinct  advancement  in 
method  that  has  been  made  was  that  involved  in  the  dis- 
covery of  the  shadow  test.  Its  importance  is  due  to  two 
main  facts:  (1)  it  is  a  measurement  of  the  complete  re- 
fraction of  the  eye,  and  (2)  it  is  an  objective  method,  not 
requiring  a  quiz  of  the  patient  as  to  the  visual  effects  of 
the  lenses  used. 

Since  its  discovery  shadow  testing  has  continued  to  be- 
come increasingly  interesting  and  popular  with  those  who 
are  engaged  in  refractive  work,  and  no  well  posted  and 
equipped  refractionist  would  think  of  ignoring  or  neglect- 
ing it.  To  the  initiate  in  this  work,  as  well  as  to  many 
otherwise  well  posted  refractionists  who  have  thus  far  dis- 
regarded the  method,  our  little  book  offers  a  means  of  be- 
coming thoroughly  familiar  with  all  of  its  practical  fea- 
tures.    It  is  put  out  for  that  purpose. 

But  shadow  testing  is  not  a  book  affair.  No  one  can  be- 
come an  expert  at  shadow  testing  by  reading  a  book  or  a 
dozen  boolvs.  It  is  an  art  and  as  an  art  the  essential  thing 
is  to  learn  to  do  hy  doing.  Unless  one  is  ready  to  lay  hold 
of  the  art  and  by  practice  acquire  dexterity  in  it,  this  book 
and  QXQYY  other  book  bearing  on  the  subject  may  as  well 
be  thrown  aside.  In  the  twelfth  chapter  a  course  of  prac- 
tice with  the  schematic  eye  is  given.  Any  one  who  follows 
the  directions  there  given,  amplifying  them  according  to 
his  need,  will  very  quickly  catch  the  whole  meaning  of  the 
test.  If  that  work  is  done  in  connection  with  reading  the 
previous  chapters,  it  will  be  all  the  better. 

In  preparing  the  manuscript  for  this  book  it  has  been 


endeavored  to  make  the  treatment  of  any  phase  of  the  test 
as  simple  as  the  test  itself.  If  the  language  at  some  points 
appears  involved  it  is  because  that  which  the  language 
attempts  to  describe  is  involved.  A  thing  cannot  be  made, 
by  any  art  of  language,  as  simple  as  it  is,  although  it  can 
easily  be  made  much  more  difficult.  If  our  language  does 
not  give  a  clear  idea  at  any  particular  point,  go  to  the 
phenomenon  itself  and  have  the  matter  thoroughly  ex- 
plained txj  you.  It  is  also  most  difficult  to  represent  in  a 
drawing  the  peculiar  light  effects  that  are  obtained  in  the 
process  of  shadow  testing  an  eye.  If  the  illustrations  do 
not  seem  quite  real,  go  to  the  test  itself  for  those  effects. 
You  can  get  them  better  there  than  anywhere  else. 


Contents 

•CHAPTER  I. 

The  Shadow  Test — General  Character  of  the  Method — 
Incident  and  Emergent  Light — The  Fundus  Reflex  and 
the  Shadows — The  ]\Iirror  and  Distance — Pupillary  Ob- 
servation— Motion  with  and  against  the  ]\Iirror — Neutral 
]\Iotion — Subsidiary  Phenomena . 

CHAPTER  II. 

Conjugate  Foci — The  Principle  of  Conjugates  in  Shadow 
Testing — Conjugate  Planes,  Retina  and  Anterior  Focal 
— Principal  and  Mongrel  Conjugates — ^Favored  Conju- 
gates in  Shadow  Testing,  40  in.,  53  in.,  27  in. — Dioptric 
Values  of  Testing  Spaces — The  Neutral  Eye  and  Non- 
neutrals — Observing  Eye  out  of  Position  and  Effects. 

CHAPTER  III. 
Physiology  of  Vision — Mobile  Factors  of  Vision — Pupillary 
Control  and  its  Purposes — Ciliary  Action  or  Accommo- 
dation and  its  Purpose — Rotation  of  the  Eyes  and  Binocu- 
lar Fusion — Natural  Stimuli — Reflex  Action  or  Spon- 
taneity— Range  of  Reflex  Activities  in  the  Eyes — 
Harmony  or  Unison  of  Ocular  Reflexes — Effects  of 
Accommodative  Action  on  Position  of  Conjugates. 

CHAPTER  IV. 

Shadow  Test  Requirements — The  Dark  Room  or  Darkened 
Space — The  Light  and  its  Position — The  ]\Iirror,  size 
and  opening — ^Use  of  Both  Eyes  by  the  Observer — Circle 


of  Illumination — ^Plane  and  Concave  Mirrors — The 
Source  of  Light — Movement  of  Source  of  Light  in  Tilt- 
ing the  Mirror — ^IMovement  of  the  Reflex,  the  same  or 
opposite  to  the  IMirror — Aerial  Image  of  the  Reflex. 

CHAPTER  V. 
Technique  in  Shadow  Testing  Emmetropia — Plane  jMirror 
at  One  Meter — Conjugates  in  Emmetropia — Patient's  Co- 
operation— Initial  Movement  in  Emmetropia — Disturb- 
ing Factors — Insertion  of  Plus  Sphericals — Reversal  of 
Motion — Expansion  of  Reflex  and  Rapidity  of  Motion — ■ 
Reduction  of  Plus  Values — Neutralization — Other  Spaces 
and  Mirrors — Mydriatics  and  Cyclopegics — Dynamic 
Skiametry. 

CHAPTER  VI. 

Technique  of  Shadow  Testing  Hyperopia — Concave  Mirror 
at  53  Inches  Distance — Initial  Appearances  and  Motion 
of  Reflex — Action  of  IMirror  and  Relation  to  Motion — 
Efi^ects  of  Hyperope  Accommodating  for  Distance — In- 
sertion of  Plus  Spherical  Values  and  Effects — Reversal 
of  Motion  and  Value  Required  for  Reversal^Subsidiary 
Phenomena:  Expansion  of  Reflex,  Rapidity  of  Motion, 
etc. — The  Neutralizing  Lens — Deduction  for  Distance, 
Correction — Use  of  Atropine,  Supposed  Advantages — 
Pheripheral  Refraction  and  Disadvantages — Myopia  and 
its  Technique — Other  Spaces  and  IMirrors  and  Effects — 
Absolute  Neutralization,  Reversal  and  Deductions  for 
Findings. 

CHAPTER  VII. 

Technique  of  Shadow  Testing  Astigmatism — Compound 
Hyperopic  Case — The  Double  Focus  for  Emergent  Light 
—The  First  Meridian,  most  Hyperopic — Second  ]Meri- 
dian,  least  Hj^peropic — Subsidiary  Phenomena  in  Astig- 
matism— The  Banded  Appearance  and  the  Straightedge 
— The  Accommodation  in  Shadow  Testing  Astigmatism 


— Different  Values  Required  for  Principal  Meridians — 
Spherical  Reductions  and  Effects — Cylindrical  Reduc- 
tions and  Effects — Concrete  Case  and  Order  of  Work — 
Spherical  Changes  and  Effects  in  high  degrees  of  Astig- 
matism—Deductions for  Distance  of  Test — Application 
of  Rule  for  Minus  Changes. 

CHAPTER  VIII. 

Technique  of  Shadow  Testing  Astigmatism  continued — The 
Second  Meridian  in  high  degrees  of  Astigmatism — Its 
Manipulation  to  Preserve  the  Banded  Appearance — 
Cylindrical  Correction  and  Elimination  of  Band — ^Re- 
verse Cylinder  and  Preservation  of  the  Band — Pro- 
ducing Band  in  Eyes  not  Astigmatic — Relation  of  Band 
to  Axis  of  Cylinder — Emphasizing  the  Band — Relation 
of  Band  to  Source  of  Light — Plane  IMirror  with  Mobile 
Light — Concave  Mirror  with  Mobile  Light — ^Application 
of  Principle  of  Conjugate  Foci  to  Mirrors — Limits  of 
both  Plane  and  Concave  Mirrors — Effects  of  Mobile 
Light  on  Band. 

CHAPTER  IX. 

Difficult  Cases — ^I'^oung  Compared  to  Older  People — Ef- 
fects of  Complexion — A  Too  Brilliant  Light  and  its 
Effects — Passivity  of  Eye  Essential  to  the  Test — Cases 
of  Photophobia — Binocular  Influences — Relation  of  Ac- 
commodation in  Two  Eyes — The  Controlling  Eye — Ani- 
sometropia in  the  Shadow  Test — Shadow  Testing  and 
Fogging  Both  Eyes  Together — Muscular  Poise  and  its 
Relation  to  Shadow  Testing — Effects  of  Imbalance,  Exo- 
phoria — Temporary  Prisms  for  Relaxation — The  Dy- 
namic Method — The  Atropine  Route — ^Peculiar  Reflexes 
— Eccentric  Motion — Zonular  Reflexes — The  Scissors 
Movement. 

CHAPTER  X. 

Shadow  Test  Devices — Frosted  Electric  Bulb — Skiascopic 


■  Chimneys — Brackets  for  Mobile  Light — Gas  Illuinina- 
tion  and  Chimney — Kerosene  and  Acetyline  Lamps — 
Mirrors  of  Various  Forms — The  Lens-Mirror — ^^The 
Schematic  Eye  and  Practice — Auxiliary  Cylinders — 
Pupillary  Disc — Skiascopes — Luminous  Eetinoscope. 

CHAPTER  XI. 

Shadow  Test  Instruments — The  Geneva  Retinoscope — 
The  Cross  Retino-Skiameter — The  Hardy  Stigmatometer. 

CHAPTER  XIL 
Practical  Training — Shadow  Testing  as  an  Art — Mechan- 
ical Devices  for  Initial  Work — Familiarity  with  Move- 
ments and  How  to  Acquire  it — Setting  Schematic  Eye 
and  Verifying  or  Correcting  it — Avoidance  of  Reflec- 
tions— Setting  Schematic  Eye  at  Known  Points  and 
Verifying — Setting  it  at  Unknown  Points  and  Determin- 
ing Findings — Deductions  and  Additions  for  Distance 
of  Test — ^Use  of  Auxiliary  Cylinders — Neutralizing  Chief 
Meridians — Producing  Banded  Appearance,  etc. — Pre- 
scription Forms  for  Findings  and  Final  Results — 
Oblique  Cylinders — Living  Subjects. 


CHAPTER  I. 

SHADOW  TESTING. 

To  be  able  to  make  an  eye,  in  any  state  of  refraction, 
reveal  its  true  condition,  without  questioning  the  patient 
as  to  what  he  can  or  cannot  see,  would  seem,  to  the  uniniti- 
ated, a  performance  little  short  of  the  marvelous.     And  yet 
this    is    precisely    what    is    done    in 
Features  shadow  testing.     While  the  requisite 

instruments  for  the  work  are  essen- 
tial, they  are  also  exceedingly  simple.  With  a  fragment 
of  broken  mirror  and  a  lighted  candle,  the  eyes  may  be 
successfully  tested  and  their  condition  found  in  this  way; 
but  it  is  of  course  better  to  have  a  mirror  with  a  small  cen- 
tral peep  hole,  and  a  light  of  sufficient  intensity  to  make 
the  appearances  perfectly  clear  and  definite. 

Shadow  testing  is  the  study  of  light  emerging  from  the 
eye  at  a  definite  view  point  before  it,  as  at  one  meter, 
though  different  distances  are  preferred  by  different  opera- 
tors.    The   distance,  whatever  it  is,  must  be  taken  into 
account  in  determining  the  correction, 
inuimnation  of        ^^^^  -^  -^  important  chiefly  for  that 

reason.  To  study  light  emerging 
from  an  eye  The  retina  must  be  illuminated  in  some  manner, 
in  order  to  provide  light  of  sufficient  intensity  to  enable 
the  observer  to  see  and  study  the  effects.  A  simple  method 
of  doing  this  is  obtained  by  using  a  mirror  in  the  manner 
shown  in  the  illustration.  The  retinal  illumination  is  of 
course  small,  for  the  action  of  the  dioptric  media  reduces 
the  area  covered,  but  it  is  sufficient.  Light  from  it  emerges 
from  the  eye,  passes  across  the  space  to  the  mirror,  and  mi- 
nute pencils  pass  through  the  peep  hole  and  are  admitted 

11 


into  the  eye  of  the  observer.  As  a  result  of  this  arrangement 
the  pupil  of  the  eye  under  observation  is  lighted  up  by  a 
sheen  of  diffuse,  reddish  light.  It  is  really  the  illuminated 
spot  on  the  retina  seen  through  the  dioptric  media  as 
through  a  lens.  Such  sheen  of  light  is  called  the  fundus 
reflex,  and  the  dark  non-illuminated  areas  surrounding  it 
are  the  shadows,  so  called. 

The  fundus  reflex  will  appear  in  the  pupil  only  when 
the  mirror  is  so  inclined  as  to  reflect  light  upon  the  ob- 
served eye,  for  then  only  is  there  a  bright  spot  on  the 
retina  to  emit  light.     The  observer's  eye  must  also  be  di- 
rectly back  of  the  peep  hole  and  in 
'^'^Be^flex^''^  line    with    the     emerging    light    to 

''catch"  the  reflex.  As  the  reflex  is 
seen  through  the  strong  lenses  that  compose  the  dioptric 
media  of  the  eye  under  observation,  it  is  apt  to  be  a  very 
** shifty"  thing,  and  often  has  to  be  ''caught  on  the  fly" 
as  it  is  entering  or  disappearing  from  the  pupil.  There 
will  also  appear  in  the  pupil  other  small  and  intense  lights, 
moving  quickly  here  and  there.  These  are  reflections 
from  dioptric  surfaces  and  are  to  be  disregarded.  The  re- 
flex proper  has  a  somewhat  fiery  appearance,  taking  its 
color  from  the  blood  vessels  of  the  retina.  At  critical 
points  in  shadow  testing  it  is  so  dissipated  as  to  appear 
like  a  pale  gray  cloud  in  the  pupil,  which  it  entirely  covers. 
Shadow  testing  is  essentially  a  "movement"  test.  It 
involves  the  determination  of  the  direction  in  which  the 
reflex  and  its  surrounding  shadows  move  across  the  pupil 
of  the  observed  eye  when  the  mirror  that  reflects  light  to 
it  is  tilted  in  a  particular  direction. 
Movements.  •^^'  whcn  the  mirror  is  tilted  in  any 

direction,  the  reflex  crosses  the  pupil 
in  the  same  direction,  motion  is  said  to  be  with  the  mirror; 
but  if  the  reflex  takes  an  exactly  opposite  course  from  the 
tilting  of  the  mirror,  motion  is  said  to  be  against  the  mir- 

12 


ror;  if  the  reflex  comes  into  the  pupil  so  quickly  that  we 
cannot  observe  from  what  direction  it  comes,  and  goes  out 
without  our  being  able  to  observe  its  direction  of  depar- 
ture, the  motion  is  neutral ^  or  neither  with  nor  against 
the  mirror.  It  is  all  but  impossible  to  get  this  effect  abso- 
lutely; but  other  effects  enable  us  to  decide  with  sufficient 
accuracy  when  motion  is  neutral.  It  is  through  these 
movements  and  the  alteration  of  effects  that  are  produced 
by  placing  trial  lenses  before  the  eye  being  examined,  that 
we  are  able  to  determine  its  condition  of  refraction,  and 
what  kind  and  the  dioptric  value  of  the  lens  the  eye  needs 
to  wear. 

Along  with  the  changes  of  movement  in  the  reflex  and 
its  shadows,  there  are  other  effects  that  help  to  make  the 
determination  accurate,  and  to  warn  us  of  approach  to 
the  neutralizing  point.     These  are  as  follows:  (1)  the  ex- 

Dansion  of  the  reflex  and  the  elimina- 
Ph?nomena.  ^ion  of  the  shadows,  (2)  its  taking  on 

a  faded  or  grayish  color,  or  the  dis- 
appearance of  the  red,  (3)  the  rapidity  with  which  the 
reflex  moves,  (4)  the  contour  of  the  line  of  separation  be- 
tween the  reflex  and  the  shadow.  With  wider  diffusion 
of  the  reflex  the  line  separating  the  reflex  from  the  shadow 
becomes  a  smaller  arc  of  a  greater  circle.  If  this  circle  of 
diffusion  were  completed  outside  of  the  pupil  it  would  be 
many  times  the  size  of  the  pupil  itself,  every  increase  in 
diffusion  extending  it,  until  at  neutralization  it  is  infinite. 


15 


CHAPTER  II. 

CONJUGATE  FOCI. 

Shadow  testing  is  based  upon  the  principle  of  conjugate 
foci.  Positive  conjugate  foci  are  two  points  so  situated 
with  reference  to  a  phis  lens  that  either  point  is  the  focus 
for  light  from  the  other  point  which  passes  through  the 

lens.     The  two  planes  of  the  foci  at 
cSuSesf  ^^^^*  angles  to  the  principal  axis  of 

the  lens  are  conjugate  focal  planes,  for 
each  point  in  either  has  a  conjugate  point  in  the  other. 
There  are  an  indefinite  number  of  such  conjugates  for 
every  plus  lens.  For  instance,  an  8  D.  plus  lens  estab- 
lishes the  conjugate  relation  (1)  for  infinity  on  either  side 
of  the  lens  with  a  plane  at  one  focal  length  from  the  lens 
on  the  other  side;  (2)  for  two  planes  each  at  two  focal 
lengths  from  the  lens  on  opposite  sides;  (3)  many  other 
distances,  as  at  13  in.  on  one  side  and  8  in.  on  the  other, 
at  20  in.  on  one  side  and  6  2/3  in.  on  the  other,  etc.  The 
first  set  of  conjugates  above  are  the  principal  conjugates; 
the  second  are  symmetrical  conjugates;  and  all  others  "are 
mongrel  or  unclassified  conjugates. 

In  shadow  testing,  the  conjugates  selected  for  practical 
purposes  depend  upon  the  distance  from  the  observed  eye 
the  operator  chooses  to  w^ork.  "Whatever  the  distance 
chosen,  its  plane  forms  one  and  the  retina  of  the  observed 

eye    the    other    conjugate.      If    the 
^^Deductions.^*       working  distance  chosen  is  one  meter, 

it  is  the  external  conjugate,  the  lens 
system  of  the  eye,  supplemented  by  whatever  lens  may  be 
required  to  bring  them  into  the  conjugate  relationship,  be- 
ing the  lens  involved.     Other  distances  often  chosen  are 

16 


at  53  in.  and  at  27  in.  The  dioptric  equivalents  of  these 
distances  are  respectively  as  follows:  for  53  in.,  0.75  D., 
for  one  meter,  1.00  D.,  for  27  in.,  1.50  D.  These  are 
therefore  the  deductions  or  additions  to  the  dark-room, 
shadow  test  findings.  Given  a  lens  of  specific  value,  if  one 
conjugate  is  fixed  the  other  is  thereby  fixed  also,  depend- 
ing upon  the  power  of  the  lens  and  the  position  of  the  first 
conjugate.  In  the  scheme  of  shadow  testing  we  locate  both 
conjugates,  one  being  the  retina  and  the  other  the  chosen 
working  distance,  and  then  by  using  supplementary  lenses 
before  the  eye  as  required  bring  the  two  into  conjugate 
relationship.  The  lens  required  to  bring  this  relationship 
about  is  then  the  key  to  the  true  condition  of  the  refraction 
of  the  eye  under  examination. 

THE  NEUTRAL  EYE. 

In  Chapter  I  we  defined  neutrality  in  shadow  testing. 
It  is  that  condition  in  which  the  reflex  fills  the  entire  pupil 
at  once  and  leaves  it  without  a  trace  of  its  direction  of  de- 
parture.    This  happens  in  only  one  condition  of  the  eye 
under  observation.     It  is  at  the  point 
Motion^  when  light  from  the  retina  of  the  ob- 

served eye  focuses  at  the  nodal  point 
of  the  observing  eye  back  of  the  peep  hole  of  the  mirror. 
Such  an  eye,  if  no  accommodation  is  involved,  is  myopic 
to  the  extent  of  the  distance  of  observation :  0 .  75  D.  for  53 
in.,  1.00  D.  for  40  in.,  1.50  D.  for  27  in.  Since  light  from 
the  illuminated  area  of  the  retina  of  the  observed  eye  is 
focused  by  that  eye  at  the  nodal  point  of  the  observing 
eye,  it  passes  into  the  latter  unrefracted,  or  precisely  as 
though  there  were  no  dioptric  factors  there.  The  light  of 
each  pencil  spreads  over  the  entire  occupied  field  of  the 
retina,  which  is  confined  of  course  to  the  field  occupied  by 
the  image  of  the  pupil  of  the  observed  eye.  There  is  noth- 
ing to  differentiate  the  diff'erent  pencils. 

19 


It  is  evident  that  from  an  eye  in  this  refractive  condi- 
tion no  effect  can  be  produced  by  tilting  the  mirror  in  any 
direction,  except  by  so  tilting  it  that  light  will  not  enter, 
and  therefore  not  emerge,  from  it.     If  it  both  enters  and 

emerges,  and  it  will  do  both  if  it  does 
^obser^er°^  One,  the  entire  pupil  will  appear  filled 

with  a  sheen  of  light,  and  veins  and 
arteries  can  only  give  a  slight  reddish  tinge  to  the  whole. 
The  same  effect  is  produced  as  when  a  small  flame  is  fo- 
cused by  a  plus  lens  upon  the  pupillary  space  of  any  eye. 
To  such  an  eye  the  flame,  however  small  or  distant,  fills 
the  entire  area  of  the  lens,  except  as  aberration  arranges 
it  in  rings  or  zones  of  fire  around  the  poles  of  the  lens. 
In  shadow  testing  an  eye,  whether  it  gives  these  appear- 
ances in  the  first  instance  or  is  made  to  do  so  by  the  im- 
position of  lenses,  is  said  to  be  neutral.  To  move  the  ob- 
serving eye  nearer  or  farther  than  such  conjugate  focal 
distance,  whatever  it  may  be,  dispels  these  neutral  effects. 
At  a  nearer  distance  motion  is  in  one  direction,  at  a  far- 
ther distance  it  is  in  the  opposite  direction.  Approach  or 
recession  of  the  observing  eye  to  or  from  the  neutral  posi- 
tion also  tends  to  cut  down  diffusion  and  to  introduce  a 
more  decidedly  reddish  cast  to  the  reflex.  The  border  lines 
between  the  reflex  and  the  shadows  also  begin  to  assume 
greater  distinctness  of  contour,  but  they  at  no  time  become 
perfectly  sharp.  It  will  be  apparent  to  those  who  are  read 
up  on  the  subject  that  neutralization  is  placing  that  which, 
in  the  parlance  of  the  text  book  writers,  is  called  the  ' '  point 
of  reversal"  at  the  nodal  point  of  the  observing  eye. 

NON-NEUTRALS. 

Since,  for  any  distance  of  observation,  a  neutral  eye 
establishes  a  degree  of  myopia  corresponding  to  the  space, 
it  is  evident  that  any  greater  or  less  degree  of  myopia 
than  the  space  calls  for  would  make  the  eye  non-neutral 

20 


for  that  space.     Myopia  of  0.75  D. 

Concave  Mirrors 


Plane  and  ^^,^^^j^  ^^^  ^^^^^^  ^^^  ^^^  neutral  for 


one  meter  nor  for  27  in.,  nor  for  any- 
other  space  than  53  in.  Nor  would  1 .  00  D.  of  myopia  be 
neutral  for  53  in.  nor  27  in.  nor  any  other  space  than  40 
in.  So  also  of  1 .  50  D.  myopia,  which  w^ould  be  neutral  for 
a  27  in.  space  only.  Of  course  mj^opia  of  a"  greater  degree 
than  any  of  these  amounts  would  be  non-neutral  for  all 
of  them.  Emmetropia  and  any  degree  of  hypermetropia 
is  of  course  non-neutral  for  all  of  them.  Whether  an  eye 
in  a  hyperopic,  mj^opic  or  emmetropic  condition  gives  mo- 
tion with  or  motion  against  the  mirror  depends  also  upon 
the  kind  of  mirror  employed.  A  concave  mirror,  which 
focalizes  the  reflected  light  before  it  enters  the  observed 
eye,  will  give  for  any  eye,  except  a  neutral,  precisely  oppo- 
site motion  from  a  plane  mirror.  This  difference  is  due 
to  the  fact  that  it  moves  the  real  fundus  reflex  in  the  ob- 
served eye  in  the  opposite  direction  from  the  way  it  is 
tilted,  or  from  that  of  a  plane  mirror.  The  reason  for  this 
is  explained  in  a  later  chapter. 


21 


CHAPTER  III. 

PHYSIOLOGY  OF  VISION. 

The  eye  is  primarily  on  organ  of  sense,  a  unit  among 
the  sensory  agencies  by  which  knowledge  of  the  external 
world  and  of  its  panorama  of  events  is  conveyed  to  the 
mind.     To  discharge  this  function  to  the  fullest  degree  it 
must  have  mobility,  and  thus  be  able 
Factors.  *  ^^  meet  the  varying  requirements  of 

changing  circumstances.  The  eyes 
possess  such  mobility,  and  chiefly  by  the  use  of  three  motor 
factors.  One  of  these  controls  the  extent  of  the  pupillary 
opening  that  admits  light  into  the  eye.  By  contraction  of 
the  sphincter  muscle  of  the  iris,  with  simultaneous  relax- 
ation of  the  radial  fibers,  the  pupil  is  made  smaller.  Two 
purposes  are  served  by  this  action:  (1)  if  the  light  is  too 
brilliant,  reducing  the  quantity  in  this  manner  tones  down 
the  images  on  the  retina;  but  (2)  as  a  diaphragm  such 
closing  of  the  pupil  also  tends  to  sharpen  the  definition  of 
the  images  by  minimizing  diffusion.  The  iris  acts  like  a 
shutter  in  a  camera. 

By  a  second  motor  factor,  the  ciliary  muscle  or  muscle 
of  accommodation,  the  lens  power  of  the  eye  may  be  in- 
creased.    Without  concerning  ourselves  as  to  the  different 
theories  of  accommodation,  it  is  accepted  by  all  that  accom- 
modation incre^es  the  curvature  of 
Accomm^odation.       ^^^  Crystalline  lens,  and  chiefly  of  its 
anterior   surface,    thereby   giving    it 
greater  dioptric  power.     This  physiological  power  is  needed 
to  adapt  the  refraction  of  the  eye  to  varying  distances  of 
the  object  viewed,  or  to  preserve  upon  the  retina  a  dis- 
tinct image  even  though  the  object  is  brought  nearer  or 

22 


remoxed  to  a  greater  distance  from  the  eye.  In  eyes  whose 
static  lens  power  is  not  sufficient  for  distant  light,  the 
accommodation  is  used  for  distance,  though  this  was  not 
evidently  the  design  in  providing  it  with  the  power.  The 
eye  accommodates  normally  with  the  approach  of  the  ob- 
ject, and  accommodation  is  normally  relaxed  with  the  re- 
cession of  the  object ;  or  the  accommodation  normally  acts 
for  near  objects  and  relaxes  for  distant  ones. 

The  third  motor  factors  are  those  that  rotate  the  two 
eyes,  or  either  of  them,  to  the  relative  position  required  for 
the  fusion  of  the  two  retinal  images,  so  that  notwithstand- 
ing the  duplication  of  the  images  on  the  two  retinae,  there 

will  be  but  one  mind  picture,  or  what 
mnocuiar  -^  termed  binocular  single  vision.  IMus- 

cles  external  to  the  eyes  cause  them 
to  perform  these  rotations.  There  are  three  pairs  of  mus- 
cles for  each  eye,  rotating  them  respectively  on  three  chief 
axes  at  right  angles  to  each  other.  But  a  given  pair  of 
muscles  rarely  acts  singly,  two  or  more  pairs  being  com- 
bined in  an  action,  such  combination  producing  rotation 
in  any  desired  direction,  or  on  any  desired  axis.  The  use  of 
these  muscles  merely  to  range  the  eyes  from  point  to  point 
is  not  strictly  a  binocular  use  of  them,  for  such  use  would 
be  necessary  in  a  single  eyed  animal.  Their  use  in  pairs, 
one  muscle  for  each  eye,  and  those  moving  the  eyes  in  oppo- 
site directions,  so  as  to  alter  the  relative  directions  of  the 
two  lines  of  vision,  is  a  binocular  use,  and  such  movement 
is  for  the  purpose  of  fusing  the  images  only. 

NATURAL  STIMULI. 

Nature  has  wisely  provided  for  the  control  of  these 
movements  without  involving  the  exercise  of  will  power. 
The  eyes  are  not  only  sensory  organs  but  they  are  respon- 
sive, not  to  the  will  but  to  physiological  stimulus.     A  de- 

23 


fective  or  anomalous  sensory  visual 

Action. 


Beflez  effect  causes  the  responsive  stimulus 


that  corrects  or  tends  to  correct  the 
defect  or  anomaly.  The  action  is  what  is  termed  in  physi- 
ology "reflex  action.".  In  a  strong  light  the  pupil  con- 
tracts automatically,  but  nerve  stimulus  and  muscular  con- 
traction are  none  the  less  required.  It  is  not  the  eye  but  ^ 
the  mind  that  impels  the  action.  We  must  take  mental 
note  of  the  excessive  light  before  the  action  to  reduce  it 
takes  place.  The  pupil  will  contract  for  a  strong  light 
only  when  the  mind  contemplates  the  brightness.  It  is  not 
even  necessary  that  the  image  fall  upon  the  macula,  al- 
though it  is  more  usual  for  those  things  that  are  held  in 
visual  contemplation  to  be  focused  upon  it,  and  it  requires 
a  special  mental  effort  to  visually  contemplate  an  image  not 
so  located. 

In  the  same  way  the  accommodation  is  aroused  to  action, 
not  by  the  nearness  of  the  object  alone,  nor  is  it  relaxed 
by  the  mere  distance  of  the  object  unless  such  object  or 
objects  are  held  in  visual  contemplation.     There  are  always 

objects  before  the  eyes,  some  of  them 
ConTempLion.      ^^^^  ^^^^  ^^me  of  them  at  a  greater 

or  less  distance.  We  can,  by  the  will, 
contemplate  either  the  near  or  distant  ones.  That  is  the 
exercise  of  a  mental  faculty.  But  when  we  visually  con- 
template the  near  object  the  accommodation  acts  involun- 
tarily so  as  to  make  a  clear  image  of  the  object  on  the 
retina,  the  distant  object  being  thrown  out  of  focus  by 
such  action.  But  we  may,  also  by  mental  process,  with- 
draw our  attention  from  the  near  object  and  visually  con- 
template the  distant  one.  This  will  involuntarily  relax 
the  accommodation,  and  thereby  the  near  object  will  be 
thrown  out  of  focus.  That  the  mind  is  a  factor  in  the  pro- 
cess, and  that  the  position  of  the  image  relative  to  the 
macula  is  not,  is  shown  by  looking  through  a  fine  wire  net- 

24 


ting,  say  13  in.  from  the  eyes,  at  distant  objects.  Both 
the  wires  and  the  distant  object,  which  are  in  the  same 
line  of  vision,  are  on  the  macula  together,  but  we  adapt  the 
accommodation  to  and  see  distinctly  only  that  which  the 
mind  contemplates,  -and  we  can,  by  will  power,  visually 
contemplate  the  one  we  choose,  or  first  one  and  then  the 
other,  without  changing  the  position  of  the  eyes. 

The  fusion  stimulus  is  of  the  same  general  character 
as  those  noted  above.  The  action  is  a  reflex  or  spontaneous 
action.  The  sensation  of  separation  of  the  images,  or  ten- 
•dency  to  separate,  spontaneously  calls  into  action  the  requi- 
site stimulus  for  fusion,  and  the  eyes 
^Reflexes.^'  are   brought    into    proper   alignment 

for  single  vision.  But  in  this  as  in 
all  physiological  actions,  nerves  and  muscles  perform  their 
allotted  part;  and  if  the  correction  of  the  anomaly  is  not 
within  the  range  of  the  power  of  the  muscles  required  to 
overcome  it,  or  if  for  any  reason  the  nervous  impulse  re- 
quired cannot  be  provided,  the  mobile  factors  succumb  to 
the  anomaly.  There  is  nothing  else  for  them  to  do.  The 
intense  light,  though  painful,  pours  into  the  eye  unhin- 
dered, unless  the  lids  are  closed  or  some  artificial  screen 
is  held  before  it;  blurred  images  are  formed  upon  the 
retina  and  vision  is  impaired,  except  as  we  are  able  to 
screen  it  down  by  the  iris  and  using  what  accommodation 
we  can ;  or  we  see  double,  two  objects  instead  of  one,  every- 
thing being  visually  duplicated.  In  the  last  case  a  partial 
correction  is  no  improvement,  and  so  obnoxious  is  it  to  the 
sense  of  vision  that  an  effort  is  made  to  suppress  vision  in 
one  of  the  offending  eyes.  This  effort  is  something  more 
than  merely  *' disregarding"  the  vision  of  one  eye,  for  it 
quickly  results  in  absolute  loss  of  visual  power  in  one  of 
the  eyes  in  a  majority  of  cases. 

These  motor  factors  in  the  eyes  are  all  more  or  less 
anited,  acting  in  unison.     A  strong  light  upon  the  eyes, 

25 


tending  to  cause  pupillary  contraction,  also  tends  to  stimu- 
late the   accommodation,   for  the  nerves   controlling  the 

sphincter  muscles  of  the  iris  are  of 
^un^Bon*"^  the  same  system  as  those  stimulating 

the  ciliary  muscles,  having  a  common 
nerve  center  in  the  brain.  But  the  muscles  that  rotate  the 
eyes  inward,  or  toward  the  nose,  are  also  stimulated  from 
the  ^me  nerve  center,  and  there  is  a  tendency  to  converge 
the  eyes  during  ciliary  action.  Conversely,  a  condition  of 
the  muscles  that  demands  excessive  activity  on  the  part  of 
the  internal  recti  muscles,  such  as  converging  the  eyes  in 
order  to  parallel  the  visual  lines  for  distance,  tends  to  in- 
cite ciliary  action  and  to  cause  contraction  of  the  pupil. 
In  this  manner  an  action  by  one  set  of  muscles  involves 
action,  or  a  tendency  to  act,  by  others,  and  complete  relax- 
ation of  any  of  the  motor  factors  depends  upon  relaxation 
of  all  of  them  together.  In  the  study  of  the  refraction 
of  the  eyes  these  influences  cannot  be  ignored,  nor,  in  the 
study  of  the  muscular  condition,  can  the  refraction  be 
treated  as  a  negligible  quantity. 

When  the  eyes  accommodate  the  position  of  the  plane 
that  is  conjugate  to  the  retina  is  brought  nearer,  for  by 
accommodation  the  refraction  of  the  eye  is  increased,  and 
since  the  retina  is  fixed  in  position,  its  conjugate,  external 

to   the   eye,   takes   a   position   corres- 
Effects  on  ponding  to  the  increased  refraction, 

conjugrates.  Thus,  if  a  normal  eye  accommodates 

0.75  D.  its  conjugate  plane  is  at  53 
in.  from  the  eye;  for  1.00  D.  accommodation  the  conjugate 
plane  is  at  1  meter;  and  for  1.50  D.  accommodation  the 
conjugate  plane  is  at  27  inches.  In  shadow  testing  the 
eyes  it  is  therefore  important  either  (1)  not  to  involve  the 
accommodation,  or  (2)  if  it  is  involved,  to  know  to  what 
extent  it  accommodates.  We  endeavor  usually  to  secure,' 
as  nearly  as  possible,  a  relaxed  accommodation;  but  there 

26 


are  advocates  of  another  system,  a  system  of  measuring 
the  refraction  during  acconmiodative  activity,  or  relaxing 
it  for  a  point  within  infinity.  This  method  has  been  styled 
'' dynamic"  skiametry.  Of  course  the  proper  deductions 
or  additions  are  made  for  the  distance  of  the  point  fixed 
by  the  observed  eye  during  such  an  examination.  Like 
every  other  mode  of  testing  the  eyes,  shadow  testing  in- 
volves all  factors  contributing  to  the  result  or  tending  to 
conceal  the  true  refraction.  It  extends  through  all  the 
dioptric  media  of  the  eyes,  and  is  therefore  a  measure  of 
the  whole  refraction,  regardless  of  the  separate  values  of 
the  cornea,  crystalline  lens,  etc. 


27 


CHAPTER  IV. 

PRIMARY    REQUIREMENTS. 

Shadow  testing,  without  special  instruments,  requires  a 
space  screened  from  general  light,  or  a  dark  room.  Other- 
wise the  phenomena  are  less  distinct  than  required.  The 
special  light  may  be  a  frosted  electric  bulb  or  a  gas  or  lamp 

flame.    Unusual  brilliancy,  such  as  is 
^for^est.^  developed  by  a  Welsbach  mantel,  is 

not  necessary,  unless  a  skiascopic 
chimney,  which  screens  all  but  a  small  area  of  the  light, 
is  also  used.  As  to  the  necessity  of  such  a  chimney,  it  is 
by  no  means  imperative,  an  ordinary  bulb  or  flame  entirely 
uncovered  answering  every  purpose.  But  an  unfrosted 
electric  bulb  is  unsatisfactory  for  the  reason  that  the  wires 
stand  out  too  prominently  and  tend  to  give  an  appearance 
of  astigmatism  whether  there  is  any  present  or  not. 

The  position  of  the  bulb  or  flame  must  be  such  that, 
while  the  light  will  fall  upon  the  mirror  and  be  reflected 
by  it  to  the  observed  eye,  it  cannot  reach  the  eye  directly 
or  by  any  different  course.  A  position  back  of  the  patient 's 

head,  and  either  above  or  to  one  side 
^ight^  of  it,  is  necessary.    The  extent  of  the 

pencils  of  light  reflected  to  the  ob- 
served eye  depends  upon  the  size  of  the  reflecting  surface 
of  the  mirror ;  but  whatever  its  size  only  pencils  of  the  lat- 
eral width  of  the  pupil  can  enter  the  eye  to  which  light  is 
reflected.  The  pencils  should  be  of  such  extent  that  the 
operator  can  readily  follow  their  course  and  easily  direct 
them  to  the  pupil.  With  a  mirror  of  from  one  to  two 
inches  in  diameter  this  can  easily  be  done.  The  reflection 
from  the  mirror  makes  a  bright  round  patch  of  light  wher- 
ever it  falls,  and  its  course  is  followed,  when  tilting  the 

28 


P.M^___— -^-^^^ 

^ 

ll-^--;:::::r3= 

^=4^.^i^ 

Direct    Tief lection. . 

'       ^ 

f|S,^-'-"",'. \ — = 

-.V'^-' rilUd    Upu^ard 

■   :          ^^-.^      ~~   -- -^__  P.M. 

^^^7?ec/  /.f^/i> 

Tilted   DoLunivard  ^\.^^ 

CM..,.--'"  ,.'"r;^ 

Upujard      \      .''    ^ — 

=^i7?eaJ  Li^ht 

CM. 

Tilted    r^^^^^^^^^ 

.1 

Mirrors  anc/  Sou/^ce  0/  L 

^/./ 

mirror,  by  noting  the  position  and  tracing  the  movements 
of  this  lighted  spot,  which  is  round  the  same  as  the  mirror. 
The  eye  that  is  not  used  in  the  shadow  test,  that  is,  the 
one  not  back  of  the  peep  hole  of  the  mirror,  is  easily  trained 
to  follow  the  movements  of  this  patch  of  light  and  help 
to  guide  it  in  the  right  direction.  Until  it  is  sufficiently 
trained  for  the  work  a  small  paper  card  may  be  held  in 
position  to  show  where  the  light  is. 

The  catoptric  principles  involved  in  shadow  testing  are 
exceedingly  simple.  A  plane  mirror  produces  a  virtual 
image  of  objects  before  it,  the  images  being  as  far  back  of 
the  mirror  as  the  objects  are  in  front  of  it.  We  may  re- 
gard this  image  as  the  ''source  of 
^^^^glT  °^  li^^*"  t^at  is  directed  into  the  pupil. 
If  the  light  is  in  a  line  perpendicular 
to  the  face  of  the  mirror,  the  virtual  image,  or  source  of 
light,  will  be  in  the  same  line  projected  back  of  the  mirror. 
But  if  the  light  is  to  one  side  of  the  perpendicular,  the 
reflected  light  goes  to  the  opposite  side,  and  this  places 
the  virtual  image  to  the  same  side  of  the  perpendicular  as 
the  light  itself.  Hence,  with  a  stationary  light,  if  the  mir- 
ror is  tilted  in  any  direction  the  virtual  image  and  source 
of  light  is  moved  in  the  opposite  direction.  Tilting  the 
mirror  downward  therefore  throws  the  source  of  light  up- 
ward, tilting  it  upward  throws  the  source  of  light  down- 
ward, and  the  same  inverse  principle  holds  true  for  tilting 
it  to  right  or  left. 

A   concave  mirror,   which   focalizes  the   light   received 

before  it  enters  the  observed  eye,  makes  a  real  image  of  the 

flame  or  bulb  at  that  focus,  and  this  image  becomes  the 

source  of  light.     Since  tilting  the  mirror  in  any  direction 

moves  this  image  in  the  same  direc- 

^^Mirror!*^^        tion,  the  source  of  light  moves  in  the 

same  direction  as  the  mirror  is  tilted, 

and  this  is  a  directly  opposite  movement,  for  the  same  tilt- 

31 


ing,  from  that  produced  by  a  plane  mirror.  Hence  the 
motion  rules  for  plane  and  concave  mirrors  are  opposite 
to  each  other.  But  neither  motion  of  either  mirror  gives 
a  definite  direction  to  the  movement  of  the  reflex,  for  the 
eye  examined  is  also  a  factor  in  the  direction  of  motion — 
the  last  determining  factor.  With  a  concave  mirror  the 
source  of  light  is  necessarily  nearer  the  observed  eye,  for  it 
is  only  a  part  of  the  space  between  the  two  eyes  from  the 
latter.  With  a  plane  mirror,  on  the  other  hand,  the  source 
of  light  is  the  sum  of  the  two  spaces,  from  light  to  mirror 
and  from  the  mirror  to  the  observed  eye,  distant.  This  is 
one  reason  why  the  plane  mirror  is  preferred  by  many  of 
the  best  refractionists.  It  is  especially  advantageous 
in  low  degrees  of  astigmatism,  where  a  distant  source  of 
light  is  needed;  but  a  concave  mirror,  which  provides  a 
near  source  of  light,  is  equally  advantageous  in  high  de- 
grees of  astigmatism. 

So  far  as  the  movement  of  the  reflex  is  concerned,  which 
is  the  principal  determining  phenomenon  in  shadow  testing, 
there  is  no  choice  between  plane  and  concave  mirror.     It 
is  only  in  the  minor  phenomena,  such  as  the  banded  ap- 
pearance  in   astigmatism,    that    one 
^rlctors'^''  may  be  better  than  the  other.  Shadow 

testing  is  "not  dependent  upon  the 
kind  of  light  nor  the  manner  in  which  it  is  reflected  to  the 
eye,  except  as  stated  above,  nor  is  the  focalizing  of  the 
light  in  the  eye  in  any  particular  manner  sought  for,  so 
long  as  it  does  enter  the  eye  and  brightens  an  area 
of  the  retina,  which  it  will  do  whether  focused  at  the  retina 
or  not.  It  is  with  the  light  emerging  from  the  eye  through 
the  dioptric  media  that  we  have  to  do,  for  that  shows  us 
the  condition  of  refraction.  If  emergent  light  is  focused 
hy  the  observed  eye  at  the  observing  eye,  motion  of  the 
reflex  is  neutral;  if  focused  beyond  the  observing  eye,  or 
negatively  back  of  the  observed  eye,  we  see  the  actual  move- 

32 


ment  that  the  reflex  makes ;  if  focused  between  the  observed 
and  observing  eye,  we  see  the  motion  of  the  aerial  image  of 
the  reflex,  which  is  opposite  to  the  motion  of  the  reflex 
itself.  The  kind  of  motion  that  is  produced  by  different 
mirrors,  at  different  distances  of  observation,  and  for  the 
different  refractive  conditions  of  the  eye  observed  may  be 
conveniently  tabulated  as  follows: 


PLANE 
MIRROR 


1.  Hypertnetropia  of  any  amount 

2.  Emmetropia 

3.  Myopia  of  less  than  O.  75  D. 


(  1.  Motion  With 

I 

1.  At  53"    i   2.  Motion  Neutral    j   1.  Myopia  of  O.  75  D. 


2.  At  40" 


L  2.  Motion  Against    \   1.  Myopia  of  more  than  O.  75  D 

1.  Motion  With 

2.  Motion  Neutral    )   1.  Myopia  of  1.00  D 


1.  Hypermetropia  of  any  amount 

2.  Emmetropia 

3.  Myopia  of  less  thon  1.00  D. 


I  3.  Motion  Against    j   1.  Myopia  of  more  than  1.00  D. 

(  1.  Hypermetropia  of  any  amount 
'  1.  Motion  With         ]  2.  Emmetropia 

(  3.  Myopia  of  less  than  1.50  D, 

3.  At  27"    ^   2.  Motion  Neutral    j   1.  Myopia  of  1.50  D. 

I  3.  Motion  Against       1.  Myopia  of  more  than  1.50  D. 


In  the  use  of  a  concave  mirror  the  only  difference  from 
the  above  is  that  where  motion  is  with,  with  the  plane  mir- 
ror, it  will  be  against  with  the  concave  mirror,  and  where 
it  is  against  with  the  plane  mirror  it  will  be  with,  with  the 
concave  mirror.  The  neutrals  are  pre- 
cisely the  same.  As  to  the  use  of  spe- 
cial instruments  for  shadow  testing, 
of  which  there  are  several  on  the  market,  we  defer  a  discus- 
sion of  their  special  qualities  and  value  to  a  subsequent 
chapter  devoted  to  that  subject  alone. 


Variation   for 
Concave  Mirror. 


33 


CHAPTER  V. 

TECHNIQUE. 

The  purpose  of  the  following  four  or  five  chapters  is 
to  describe  in  detail  the  technique  of  practically  fitting 
eyes  by  the  shadow  test,  taking  cases  in  the  following  order : 
(1)  emmetropia,  (2)  hypermetropia,   (3)  myopia,   (4)  as- 
tigmatism.    While  shadow  testing  is 
condiMons.  ^  Valuable  method  when  used  simply 

to  corroborate  the  trial  case  or  sub- 
jective findings,  we  will  assume  it  to  be,  in  the  following, 
the  initial  method,  and  that  its  findings  are  corroborated 
by  the  trial  lenses.  We  introduce  emmetropia  into  the 
series  not  because  emmetropes  are  frequent  patients  but  be- 
cause emmetropia,  being  the  normal  or  model  condition  of 
refraction,  is  the  standard  of  physiological  action  of  the 
eyes. 

EMMETROPIA. 

Emmetropia  may  be  defined  as  that  condition  of  the  re- 
fraction of  an  eye  in  which,  without  accommodative  action, 
light  from  a  distant  source  (infinity  =  o  o  ="20  ft.  or  far- 
ther)  is  focused  upon  the  retina,  producing  sharply  de- 
fined images  of  such  distant  objects; 
conditicm^  and  which  for  all  points  nearer  to  the 

eye  than  infinity  requires,  in  order 
to  make  the  images  clear,  accommodation  in  proportion  to 
the  dioptric  space  separating  the  object  from  the  eye;  or 
which,  failing  to  accommodate  for  the  required  (normal) 
amount,  focalizes  the  light  back  of  the  retina  and  there- 
fore produces  blurred  images  upon  it.  In  emmetropia, 
since  no  accommodation  is  required  for  distance,  to  relax 
the  accommodation  all  that  is  necessary  is  to  have  the 
patient  fix  a  distant  object,  or  merely  to  look 'into  space 
without  seeing  any  definite  thing.     Light  emerging  from 

34 


an  emmetropic  eye,  in  such  state  of  relaxation,  emits  plane 
waves  or  parallel  rays  of  light.  The  retina  and  infinity 
are  conjugate  to  each  other.  Suppose  our  case,  but  that 
fact  unknown  to  us  at  first,  is  an  emmetrope. 

We  seat  the  patient  in  the  dark  room  in  such  position 
that  the  light  to  be  used  for  shadow  testing  is  back  of  her 
head,  but  either  sufficiently  above  it  or  to  one  side  to  allow 
its  light  to  fall  upon  the  mirror  when  held  in  the  position 
shown  in  the  illustration,  Chapter  I. 
^for*Te*sr*  We  take  our  position  also  as  shown 
in  the  figure.  The  patient  is  direct-, 
ed  to  look  at  some  object,  perhaps  dimly  seen  back  of  the 
operator,  for  all  light,  except  that  from  the  special  light 
used  in  the  test,  is  excluded,  or  at  least  the  space  is  made 
reasonably  dark.  We  so  incline  the  mirror  that  the  re- 
flected light  from  it  falls  upon  the  patient's  face,  and  tilt 
the  mirror  so  as  to  cause  the  light  to  pass  over  first  one 
eye  and  then  the  other.  Suppose  we  are  using  a  plane 
mirror  and  our  distance  is  one  meter  from  the  patient. 
As  the  light  falls  upon  either  eye,  we  notice  the  red  reflex 
spring  into  the  pupil.  The  patient  is  apt  at  this  point  to 
look  at  the  light  seen  in  the  mirror.  If  she  does  the  reflex 
disappears.  We  direct  her  to  look  at  the  distance  again, 
at  a  card  or  object  on  the  wall  so  placed  that  her  line  of 
vision  is,  for  her  right  eye,  just  past  the  operator's  right 
ear,  or  for  the  left  eye  past  the  left  ear,  although  both 
eyes  can  be  tested  when  looking  either  way. 

Having  properly  fixed  her  vision  upon  distance  again, 
we  so  tilt  the  mirror  as  to  cause  the  reflected  light  to  sweep 
across  one  eye,  and  as  it  crosses  that  eye,  we  note  whether 
the  reflex,  and  its  surrounding  shadows,  cross  the  pupil  in 
the  same  or  in  the  opposite  direction. 
"^^^Tes^  *^®         Being  an  emmetrope,  if  the  accommo- 
dation is  relaxed  the  reflex  will  cross 
the  pupil  in  the  same  direction  as  the  light ;  motion  will  be 

37 


with  the  mirror.  The  other  eye  will  show  the  same  motion. 
We  tilt  the  mirror  vertically,  up  and  down;  and  horizon- 
tally, to  right  and  left;  or  obliquely,  and  everywhere  get 
motion  with  the  mirror.  We  know  that  our  patient's  eyes 
are  either  less  than  1  D.  myopic,  emmetropic  or  hyper- 
opic.  The  conjugate  of  the  patient's  retina  cannot  be  be- 
tween her  eye  and  ours. 

Notice  the  proviso  above,  "if  the  accommodation  is  re- 
laxed. ' '  This  is  put  in  for  the  reason  that,  if  the  patient  vis- 
ually contemplates  the  light,  though  not  looking  directly  at 
it,  reflex  action  will  cause  the  pupil  to  constrict,  and  it  will 
tend  to  accommodate  in  spite  of  its 
CompUcaSons  emmetropia  and  fixation  of  the  dis- 

tant object.  Some  people  are  very 
difficult  to  shadow  test  on  this  account.  There  are  also 
conditions  of  the  muscles  tending  sympathetically  to  excite 
the  accommodation  that  require  attention  before  full  cili- 
ary relaxation  is  possible,  despite  the  fact  that  the  eyes 
are  emmetropic.  These  conditions  are  often  regarded  as 
pointing  to  a  spasmodic  action  of  the  ciliary,  but  are  ac- 
counted for  by  muscular  imbalance.  Exophoria  is  one  of 
those  imbalances  tending  to  induce  ciliary  action.  To 
eliminate  these  causes  for  ciliary  activity  may  be  difficult, 
but  a  temporary  prism,  base  in,  over  one  eye  will  tend  to 
relieve  the  muscular  influence  during  the  test.  However, 
some  eyes,  because  of  the  above,  or  because  of  an  exceeding 
darkness  of  the  reflex,  cannot  be  successfully  tested  in  this 
manner,  except  perhaps  by  the  most  expert. 

But,  granted  no  such  factors  interfere  with  the  test,  we 

have  decided  motion  with  the  mirror.     This,  with  the  plane 

mirror,  calls  for  plus  lenses.     We  adjust  a  pair  of  +2.00 

D.  spheres  in  a  trial  frame  made  for  the  purpose,  and  again 

make  the  test  as  before.     Reflections 

^^Kst**  °^  from  the  lenses  are  easily  avoided  by 

moving  slightly  to   one  side  of  the 

position  that  gives  them.     With  the  -f^.OO  D.  motion  in 

38 


JEin  m  ef'rooi  a 

1 

06se/'i^//7t^   Eye 

ryect  Lye 

^^-^''""^ 

~Hh 

^ 

Inci'denf    Lt'q/if 
-P/ane  Mirror 

^ 

/0^^ 

,'<' 

^ 

^/^^^ 

y~ 

^v^.y 

vv- 

^ 

V_>^ 

^ 

/^^?^^ 

Xi^ 

fcr-Mi] 

' ~~~~— ^=-1-=— — 

\ 

■---^ 

yy- 

/fof/on  /J^a/ns^ 

4i4t 

^ 

--e- 

y 

-iff- 

'\\ 

\'^^^^~~~ — 

BS^A 

.^ 

A/of/on  ^Gainsit 

^ 

irff 

~~0\ 

Afo;^/6^7    /Veul^ral 

4!^ 

^y 

^/ane  Atirror  of  one  Ne^er- 

each  eye  is  decidedly  against  the  mirror.  That  is,  light 
coming  from  the  illuminated  spot  on  the  retina  focuses 
on  its  way  to  the  observing  eye,  forming  an  aerial  image 
of  the  fundus  at  the  focus.  It  is  the  motion  of  this  image, 
which  is  opposite  to  that  of  the  real  fundus  reflex,  that  we 
watch,  and  therefore  get  reversed  motion.  Reduce  the  lens 
to  -f-1.50  D.  and  motion  is  still  against.  We  notice  also 
that  the  reflex  seems  to  cover  a  larger  area  and  to  more 
completely  fill  the  pupil ;  it  also  moves  on  and  off  the  pupil 
with  greater  rapidity. 

A  reduction  to  +1.25  D.  augments  the  effects  last  named 
— enlarges  the  reflex  and  makes  it  move  more  rapidly — 
but  gives  motion  against  the  mirror  still.     The  reflex  also 
has  a  less  reddish  cast,  as  though  the  color  had  been  dissi- 
pated through  the  larger  area,  and  is 
^McTt^on^^  taking  on   more   of   a  pale    grayish 

color.  I  cut  my  lens  down  to  +0 .  75  D., 
making  a  drop  of  one-half  diopter,  and,  while  motion  is  not 
made  more  rapid,  nor  is  the  reflex  apparently  altered  in 
extent,  motion  has  reversed,  and  the  reflex  now  goes  with 
the  mirror.  With  +1.25  D.  I  have  motion  against,  with 
+0.75  D.  I  have  motion  with.  Evidently  my  neutralizing 
lens  lies  somewhere  between  the  two.  I  substitute  a  +1.00 
D.  and  I  get  very  nearly  the  neutral  effects,  but  I  think 
motion  is  still  slightly  with  the  mirror,  and  I  am  unsatis- 
fied. Measuring  my  distance,  I  find  I  am  36  inches  from 
the  eyes,  instead  of  40  inches ;  moving  back  to  the  required 
distance  I  find  as  near  absolute  neutralization  as  I  am  likely 
to  get.  The  whole  pupil  is  illuminated  at  once,  no  matter 
in  what  direction  the  mirror  is  tilted,  and  it  entirely  passes 
off  the  pupil  in  the  same  manner.  In  this  condition  the 
slightest  touch  of  the  circle  of  light  upon  the  pupil  fills  it 
with  the  sheen  of  light,  and  it  stays  in  the  pupil  until  the 
last  edge  has  passed  off. 

The  eyes  have  been  made  to  focus  at  one  meter  by  a 

41 


+1.00  D.  spherical  lens.  Without  that  lens  light  from  the 
retina  will  focus  at  infinity  and  vice  versa.  The  retina  is 
conjugate  to  infinity  and  infinity  is  conjugate  to  the  retina, 

for  a  +1.00  D.  sphere  establishes  the 
^prnding-^^  Conjugate  to  the  retina  at  one  meter 

in  front  of  the  eye.  The  eye,  and 
both  of  them,  is  therefore  emmetropic.  The  dark  room 
finding  at  one  meter  is  +1.00  D.  sph.  The  correction  is 
1  D.  less,  or  absolutely  no  lens  at  all,  or  a  piano.  At  53 
in.  a  +0.75  sph.  would  neutralize  motion ;  at  27  in.  a  +1.50 
sph.  would  do  the  same.  With  a  concave  instead  of  a  plane 
mirror,  at  one  meter  initial  motion  would  be  against  the 
mirror;  +2.00  D.  would  make  it  with;  +1.50  D.  with; 
+1.25  D.  with;  +0.75  D.  against.  But  a  +1.00  D.  would 
neutralize  the  eye.  These  results  would  be  obtained  from 
an  emmetropic  eye  only. 

It  is  seen  that,  at  the  critical  point  in  the  test— that  is, 
when  motion  is  neutralized — the  eye  being  tested  is  fogged 
for  distance  by  a  plus  lens.  It  is  endeavored,  in  this  way, 
to  relax  the  accommodation  at  this  point,  and,  so  far  as 

fogging  the  eye  is  capable  of  relaxing 
^ydriasis^'  accommodation,   it  will    be    relaxed. 

Many  practitioners  in  the  medical 
profession  prefer  to  be  fully  assured  of  this  fact,  and  em- 
ploy a  cycloplegic  for  the  purpose.  This  also  enlarges  the 
pupil  and  gives  a  wider  field  of  view  of  the  reflex.  But 
this  fact  is  not  devoid  of  disadvantage.  By  admitting  the 
extra-visual  areas  of  the  crystalline  lens  into  the  test,  a 
somewhat  different  refraction  is  obtained  than  when  the 
test  is  applied  to  the  visual  areas  (the  normal  physiological 
pupil)  alone.  It  is  also  generally  admitted  that  the  test 
under  thorough  cycloplegia  is  not  accurate,  but  that  de- 
ductions are  required  for  the  findings  under  those  circum- 
stances, and  there  is  no  certainty  among  those  who  use 
the  drug  as  to  •the  amount  of  the  deduction. 

42 


Dynamic  shadow  testing  is  the  opposite  of  mydriasis  and 
cycloplegia.  It  engages  the  accommodation  for  a  definite 
amount  by  having  a  card  upon  the  operator's  forehead,  or 
elsewhere,  for  the  patient  to  look  at  (visually  contemplate) 

during  the  test.  If  the  patient  is 
aShod*^  able  to  accommodate  for  the  requisite 

amount,  such  card  becomes,  by  ac- 
commodation, conjugate  to  the  retina  of  the  observed  eye, 
and  since  the  observing  eye  is  practically  in  the  same  plane, 
the  eye  examined  is  neutral  for  the  distance.  Increasing 
plus  lenses  are  then  imposed  until  motion  is  reversed.  At 
one  meter  a  +1.00  sph.  should  be  the  neutralizing  lens — 
neutralizing,  in  this  case,  the  accommodation  previously 
engaged.  It  is  claimed  by  some  refractionists  that  in  this 
manner  latent  hypermetropia  is  uncovered.  Mr.  A.  J. 
Cross  of  New  York  is  the  originator  and  principal  advocate 
of  this  method. 


45* 


CHAPTER  VI. 

CONCAVE    MIRROR 53    IN. 

It  was  originally  supposed,  in  shadow  testing,  that  the 
position  of  the  "source  of  light"  was  a  factor  in  the  mo- 
tion of  the  reflex.     Accordingly,  under  this  theory,  a  con- 
cave mirror  of  10  in.  focus  held  before  the  observer 's  eye 
would  focus  at  about  13  in.  before  the 

53  in    space.         mirror;  or  in  a  space  of  53  in.  at  40 
Origrin  of  ■  ^ 

in.  from -the  observed  eye.   This  would 

place  the  source  of  light  40  in.  from  the  eye  being  exam- 
ined. The  theory  undoubtedly  led  to  the  use  of  a  testing 
space  of  53  in.  It  is  employed  exclusively  by  many  opti- 
cians. But  the  position  of  the  source  of  light  has  no  effect 
upon  the  direction  of  the  motion.  Motion  of  the  patch  of 
light  in  the  eye  is  always  exactly  opposite  to  the  motion 
of  the  source  of  light.  But  with  a  concave  mirror  the 
source  of  light  goes  in  the  same  direction  as  the  mirror 
is  tilted,  and  the  reflex  therefore  goes  in  the  opposite  direc- 
tion, and  so  appears  to  the  observer  in  emmetropia.  Hence, 
if  the  observed  eye  focuses  nearer  than  53  in.,  the  aerial 
image  of  the  reflex  goes  opposite  to  the  motion  of  the  real 
reflex,  and  appears  to  move  in  the  same  direction  that  the 
mirror  is  tilted.  This  accounts  for  the  reverse  effects  of 
using  plane  and  concave  mirrors. 

HYPERMETROPIA. 

We  will,  in  this  case,  suppose  the  observed  eye  to  be 

1.50  D.  Hyperopic,    and  that  it  accommodates    for    the 

amount  in  the  dimly  lighted  dark  room  when  the  eyes  are 

fixed  on  objects  back  of  the  operator ;  but  that,  on  account 

of  the   hyperopia,   the  patient   com- 

Hyperop^a.  plains  of   eye   strain.     We   seat   the 

patient     in     the      proper      position 

with  reference  to  the  light  and  take  our  position  at  53 

46 


in.  with  a  concave  mirror.  Reflecting  light  upon  the 
eye  we  get  motion  against  the  mirror.  That  is,  emergent 
light  focuses  beyond  the  observer 's  eye  and  the  true  motion 
of  the  reflex  is  seen.  A  +1.00  sph.  is  imposed,  but  since 
that  merely  relaxes  the  accommodation  to  that  extent,  no 
change  is  seen,  either  in  the  direction  of  motion  or  expan- 
sion of  the  reflex  or  greater  rapidity  of  motion.  A  +2.00 
D.  sph.  relaxes  all  accommodation  and  actually  causes  the 
focus  of  emergent  light  to  approach  the  observer,  but  it  is 
still  back  of  him.  There  is  therefore  no  change  in  the  di- 
rection of  motion,  but  the  reflex  expands  considerably  and 
the  motion  is  quite  rapid.  The  dissipation  of  the  color 
also  shows  that  we  are  approaching  neutralization.  A 
+2.50  sph.  is  substituted,  and  this  reverses  motion  making 
it  with  the  mirror.  Since  the  2.50  sph.  is  a  1.00  D.  over- 
correction of  the  hyperopia,  it  focuses  the  observed  eye 
at  40  in.,  and  this  is  forward  of  the  position  of  the  observ- 
ing eye.  It  is  the  aerial  image  of  the  reflex  that  now  comes 
into  view,  and  its  motion  is  the  opposite  of  that  of  the  true 
reflex.  Reducing  our  lens  to  a  +2.25  sph.  we  obtain  the 
nearest  to  neutralization  it  is  possible  to  get.  All  of  the 
phenomena  of  neutrality  are  atjbheir  best — enlarged,  pale 
reflex  and  very  quick  motion. 

Since  a  +2.25  sph.  has  neutralized  the  observed  eye,  with 
the  lens  imposed  light  from  the  retina  of  the  observed  eye 
emerges  and  focuses  at  53  in.,  the  testing  distance,  or  at 
the  observing  eye.     It  has  therefore  been  made,  by  the 

lens,  0.75  D.  myopic.    To  correct  the 
^pind^g-"^  eye,  or  make  it  normal  for  distance, 

would  require  0.75  D.  less  plus,  or 
+1.50  D.  It  is  seen  that  with  the  concave  mirror  at 
53  in.  the  only  differences  to  be  observed  from  the  plane 
mirror  test  at  40  in.  are:  (1)  the  directions  of  motion  are 
reversed,  and  (2)  the  deduction  or  addition  is  0.75  D.  in- 
stead of  1.00  D.     The  phenomena  of  enlarging  the  reflex, 

49 


increasing  the  rapidity  of  motion,  and  the  dissipation  of 
color  are  precisely  the  same.     The  difference  between  hy- 
peropia and  emmetropia  only  result  in  requiring  more  plus 
to  produce  neutralization,  for  until  the  accommodation  is 
relaxed  the  observed  eye  will  continue  to  focus  at  infinity. 
In   a   case   of  hyperopia  where  the   accommodation   is 
unable  to  hold  infinity  in  focus,  whether  it  be  because  of 
excessive  hypermetropia  or  a  lack  of  accommodation,  mo- 
tion will  be  correspondingly  slow  for  the  amount  of  such 
insufficiency    of  the   refraction.     Or 
^Mouon.^^  if  atropine  is  used  and  the  accommo- 

dative function  is  thereby  suspended, 
the  motion  is  then  slow  at  the  beginning  but  increases  in 
rapidity  with  the  imposition  of  plus  lenses  until  the  point 
of  neutralization  is  reached.  This  is  no  particular  advan- 
tage since  lenses  will  ordinarily  relax  the  accommodation 
before  you  can  arrive  at  the  point  of  neutralization.  The 
seeming  advantage  of  a  large  pupil  is  also  nullified  by  the 
fact  that  upon  central  areas  you  are  likely  to  have  motion 
contrary  to  that  which  appears  in  the  periphery  of  the 
pupil.  It  is  because  of  these  contradictory  phenomena 
that  many  who  use  atropine  also  employ  a  pupillary  disc 
cutting  out  the  peripheral  effects.  The  advantage  of  atro- 
pine, if  there  is  any  advantage,  is  thereby  reduced  to  its 
cycloplegic  effects  entirely. 

MYOPIA. 

If  our  patient  is  a  myope  for  an  amount  in  excess  of  the 

dioptric  value  of  our  testing  space,  as  for  example  3.50  D., 

the  initial  motion  for  a  concave  mirror,  without  lenses,  is 

with  the  mirror.     That  is,  motion  of  the  mirror  in  any 

direction,   as  downward,    causes  the 

Myop^f  true  fundus  reflex,  or  lighted  area  of 

the   retina,   to   move  upward.     But, 

since   light  from  this  reflex  emerges  and  focuses  at  about 

50 


111/2  in.  from  the  observed  eye,  the  aerial  image  of  the 
fundus  will  move  downward — in  the  same  direction  as  the 
mirror  is  tilted.  And  this  motion  shows  to  the  observer 
that  the  focus  of  the  observed  eye  is  forward  of  the  observ- 
ing eye,  and,  that  being  the  case,  a  minus  lens  of  the  cor- 
rect value  will  bring  it  to  the  observing  eye.  We  impose 
a  — 1.00  sph.  and  motion  is  still  with;  a  — 2.00  sph.  also 
gives  motion  with;  but  a  — 3.00  reverses  motion,  and  is 
therefore  too  strong  for  the  purpose.  Reducing  it  to  — 2.75 
sph.  neutralizes  motion,  or  focuses  the  observed  eye  at  53 
in.,  our  position.  This  lens  leaves  the  eye  0.75  D.  my- 
opic. Hence  a  lens  0.75  D.  stronger  than  the  neutralizing 
lens,  or  — 3.50  D.,  is  the  required  correction.  We  add  to 
the  minus  lens  that  is  found  to  neutralize  motion  the  diop- 
tric value  of  the  testing  distance,  in  this  case  0.75  D.  This 
is  the  same  as  subtracting  an  equal  amount  of  plus. 

Some  practitioners  do  not  attempt  absolute  neutraliza- 
tion, but  stop  with  the  weakest  lens  that  reverses  motion 
from  the  original  direction.  To  compensate  for  such  in- 
accuracy in  the  case  of  minus  findings,  instead  of  adding 

0.75  D.  the  addition  is  cut  a  quarter 
^°F^ding^.  °^         ^^  ^  diopter,  making  the  addition  for 

53  in.  0.50  D.  This  method  gives  the 
operator  the  distinct  phenomenon  of  reversal  to  go  by  in- 
stead of  the  elusive  one  of  neutralization.  It  is  on  the  same 
principle  as  if  the  marksman  set  his  forward  sight  low  so 
as  to  shoot  a  little  above  the  mark  and  then  aimed  his  rifle 
low  so  as  to  counteract  that  effect.  It  is  always  well  to 
reverse  the  initial  motion,  whatever  it  may  be,  for  then 
only  is  one  sure  of  having  passed  the  apex  of  correction. 
But  if  he  has  only  passed  it  an  eighth  of  a  diopter,  and 
such  a  small  value  will  sometimes  produce  distinct  reversal, 
the  addition  of  0.50  D.  will  not  be  quite  sufficient,  although 
it  is  on  the  safe  side  of  a  myopic  correction.  It  would 
seem  to  us  just  as  essential  to  make  the  deduction  0.50  D. 

51 


for  plus  findings,  admitting  that  the  lens  finally  used  is  a 
slight  overcorrection,  which  it  would  be  if  motion  is  re- 
versed.    Such  a  deduction  would  also  be  on  the  safe  side. 

27   IN.    SPACE. 

The  reason  for  employing  this  space,  or  even  a  shorter 
one,  is  that  it  brings  the  observer  near  enough  to  the  pa- 
tient to  make  any  changes  in  the  lenses  called  for  without 
getting  up,    and   thereby   lessens   the   danger   of   altering 
the  uniform  distance  of  making  the 

ference  between  it  and  other  spaces 
is  the  amount  of  the  deduction  or  addition  from  the  dark 
room. findings,  which  is  1.50  D. ;  or  for  a  20  in.  space  2.00 
D.  The  shorter  spaces,  because  of  nearness  to  the  observed 
eye,  give  a  greater  visual  angle  to  the  pupil,  thereby  en- 
larging the  field  of  observation.  The  phenomena  of  neu- 
tralization are  the  same  for  all  spaces,  all  mirrors  and  all 
distances,  as  are  also  the  phenomena  of  approach  to  the 
point  of  neutralization. 


52 


CHAPTER  VII. 

ASTIGMATISM. 

In  considering  the  diagonsis  of  astigmatism  by  the 
shadow  test  it  will  not  be  necessary  to  differentiate  between 
the  use  of  different  mirrors  and  different  testing  spaces. 
These  things  are  questions  of  individual  preference  merely, 

and  one  may  become  skillful  in  the 
A^r^matirm  practice  of  shadow  testing  with  the 

mirror  and  space  of  his  own  choosing. 
Whatever  space  he  chooses  to  employ  is  a  matter  of  conse- 
quence to  nobody  but  himself.  By  any  of  the  methods 
there  is  one  point  at  which  all  agree — the  point  of  neutral- 
ization— and  reversal  of  motion  is  the  same  by  one  method 
as  another.  At  the  point  of  neutralization  the  reflex  is 
greatly  expanded  ancl  diffuse;  at  either  side  of  this  point 
motion  is  very  quick;  and  with  neutralization  the  reflex 
assumes  the  faded  out  or  colorless  appearance.  Of  these 
subsidiary  phenomena,  the  expansion  of  the  reflex  is  one 
of  the  surest  signs  of  astigmatism. 

The  characteristic  feature  of  astigmatism,  of  whatever 
kind,  is  the  double  focus.  One  meridian  of  the  eye  pos- 
sessing the  highest  dioptric  value  and  the  other  at  right 
angles  to  it  having  the  lowest,  establishes  two  focal  points, 

one  for  each  meridian,  within  the  eye, 
^^pocu^^^  for  light  from   any  object.     But  if, 

by  the  mirror  reflection  employed  in 
shadow  testing,  a  bright  spot  of  light  is  produced  on  the 
retina,  light  from  it  will  emerge  from  the  eye  and  have 
also  two  points  of  focalization.  The  retina,  instead  of  be- 
ing conjugate  to  only  one  plane  in  front  of  it,  is  conju- 
gate to  two  such  planes,  the  space  between  them  being  the 

53 


so-called  interval  of  Sturm.  The  two  principal  meridians 
are  of  different  refractive  value  for  emergent  as  well  as 
incident  light.  It  is  evident  therefore  that,  in  shadow  test- 
ing such  an  eye,  the  observing  eye  cannot  be  at  both  of 
these  focal  points  at  the  same  time,  and  that,  if  it  is  at  one 
of  them,  the  other  will  necessarily  be  back  or  in  front  of 
the  observing  eye. 

To  neutralize  one  meridian  of  an  astigmatic  eye  there- 
fore leaves  the  other  principal  meridian  out  of  focus  and 
un-neutralized.  Consequently,  in  the  neutralized  meridian, 
you  will  get  all  the  phenomena  of  neutralization,  while  in 
the  opposite  meridian  motion  remains 
Meridians  distinctly  with  or  against  the  mirror, 

as  the  case  may  be.  The  problem  in 
the  shadow  testing  of  an  astigmatic  eye  is  to  determine  the 
value  of  each  principal  meridian  and  thereby  get  also  the 
difference  between  them,  and  thus  measure  the  whole  de- 
fect. A  cylindrical  value  will  be  required  to  equalize  the 
difference  between  the  meridians,  and  a  spherical  lens  will 
correct  the  uniform  error  after  such  difference  is  deter- 
mined and  corrected.  The  determination  of  the  difference 
is  the  more  difficult  part  of  the  problem,  but  in  some  re- 
spects this  is  even  easier  than  measuring  the  simple  spheri- 
cal part  of  the  error,  for  there  are  distinctive  phenomena 
for  it. 

While  shadow  testing  is  essentially  a  motion  test,  the 
subsidiary  phenomena  to  which  we  have  referred  become  of 
great  importance  in  astigmatic  cases,  especially  the  phe- 
nomenon of  the  expansion  of  the  reflex  along  the  neutral- 
ized meridian.     A  round  rubber  disc 
stSSht^dge.  ^^  equally  curved  at  all  points  of  its 

circumference,  but  if  it  is  stretched 
by  pulling  at  two  opposite  sides,  the  edges  between  assume 
practically  a  straight  line,  or  take  the  form  of  a  straight 
edge.       This  effect  gives  to  the  reflex,  in  contrast  to  the 

64 


Conjj.  Hyp.   /iJih^ mutism 


Jnctclen}    Lt^h> 
^Jain  Mirror- 1  Meter 


meri^eni     Liqfit 
Motion   h/ith 


shadowed  areas  that  surround  it,  what  is  known  as  the 
** banded"  appearance.  The  band  or  straight  edge  extends 
in  the  direction  of  the  neutralized  meridian.  There  is  no 
distinct  motion  either  with  or  against  the  mirror  if  tilted 
in  that  meridian.  The  reflex  flows  across  the  pupil  in  one 
continuous  stream,  and  has  no  defined  end.  But  when  one 
meridian  is  so  neutralized  the  other  will  show  distinct  mo- 
tion either  with  or  against,  and  by  tilting  the  mirror  in 
that  direction  the  straight  edge  crosses  the  pupil. 

If  the  vertical  meridian  of  the  eye  is  neutralized  the  band 
and  straight  edge  will  extend  vertically.  "When  the  mirror 
is  tilted  vertically  the  reflex  will  "flow"  along  that  me- 
ridian without  a  distinct  ending,  and  the  problem  then 

is  to  find  what  reduction  or  increase 
Tf 'BaneT  ^^  ^^^  ^^^^  power  is  required  to  cause 

the  band  to  assume  the  horizontal 
position  and  show  the  horizontal  straight  edge.  As  there 
are  but  two  chief  meridians,  it  cannot  be  made  to  assume 
any  other  direction  than  those  of  such  two  chief  meridians, 
be  they  vertical  and  horizontal  or  oblique,  and  whatever 
meridian  is  made  neutral  or  to  show  the  straight  edge,  the 
other  or  opposite  meridian  will  show  motion.  To  get  these 
effects  successively,  first  for  one  meridian  and  then  for 
the  other,  the  lens  changes  must  all  be  spherical,  one  value 
being  used  for  the  highest  meridian,  the  other  for  the  low- 
est, for  a  proper  cylinder  with  its  axis  along  the  neutral 
meridian  will  make  all  meridians  neutral  and  dispose  of 
the  band.  But  working  with  sphericals  alone  exposes  the 
test  to  the  danger  of  accommodative  action,  as  may  be  seen. 
One  focal  point  in  astigmatism  is  iiecessarily  back  of 
the  other.  It  is  either  nearer  to  the  retina  when  both  are 
forward  of  it,  back  of  it  when  either  is  back  of  it,  and  far- 
ther back  if  both  are  back  of  the  retina.     All  spherical 

57 


changes  in  the  lenses  allow  these  foci 
Ac^commod\«on.  ^^  remain  apart  and  put  them  in  one 
of  the  above  relations  to  each  other. 
Now,  the  accommodation  resists  the  focalizing  of  any  me- 
ridian^ back  of  the  retina.  Wherever  there  is  a  tendency 
of  even  one  focus  to  go  back  of  the  retina,  the  accommoda- 
tion becomes  active  and  holds  the  focus  from  such  retro- 
gression. Hence,  when  in  shadow  testing,  we  endeavor  by 
a  spherical  reduction  to  neutralize  the  meridian  of  greatest 
refraction,  which  is  focalized  forward  of  the  other,  and 
the  astigmatism  is  of  a  greater  dioptric  value  than  the  di- 
optric equivalent  of  the  testing  space,  we  arouse  the  resis- 
tance of  the  accommodation,  which,  acting  equally  for  all 
meridians,  prevents  neutralization  of  the  second  meridian, 
at  least  until  a  sufficient  reduction  is  made  to  overcome  the 
resistance  of  the  accommodation.  This  is  apt  to  give  the 
impression  that  there  is  a  great  deal  of  astigmatism  where 
there  is  in  fact  but  little. 

But  the  employment  of  cylinders  to  neutralize  the  sec- 
ond meridian  after  the  first  has  been  neutralized,  takes 
away  the  distinctive  banded  appearance  and  straight  edge, 
which  is  so  useful  in  testing  astigmatic  eyes.  Either  of 
the  methods  therefore  has  certain  dis- 
^^^''^Band.^''^  advantages  in  shadow  testing.  The 
problem  of  avoiding  both  difficulties 
is  not  so  hard  however  as  it  would  seem.  As  to  the  prac- 
tical use  of  such  a  method  we  will  deal  with  that  in  a  con- 
crete case.  In  such  case  we  will  specify  no  special  form 
of  mirror  nor  distance,  but  refer  to  the  case  as  ' '  requiring 
+  "  or  "requiring  — ."  In  all  cases  it  is  important  not 
to  give  either  meridian  a  focal  foothold  on  the  retina  with 
which  to  resist*  further  reduction  of  the  lens.  This  prin- 
ciple is  of  equal  importance  in  shadow  test  instruments  in 
which  the  reductions  are  spherical. 

58 


CONCRETE  CASE. 

Suppose  our  case  is  one  of  compound  hyperopic  astigma- 
tism, showing  eventually  that  a  correction  of 
+1.50  sph.=+0.75  cyl.  ax  90,  or 
♦  +2.25  sph.=— 0.75  cyl.  ax  180 

is  the   needed   correction.       At   the 
~        Testing  chosen  distance  and  with  the  chosen 

AstigrmatiBm. 

mirror  we  reflect  light  upon  the  eye 
and  the  motion  indicates  that  plus  is  required  in  all  merid- 
ians^  no  definite  marks  of  astigmatism  being  at  first  ap- 
parent. We  impose  increasing  plus  spheres  until  motion 
is  opposite  from  the  initial  motion  in  all  meridians.  "We 
then  reduce  gradually  until  the  horizontal  meridian  (the 
one  requiring  the  greater  amount  of  plus)  is  decided  to  be 
neutral.  This  would  be  in  the  above  case,  for  53  in.,  +3.00 
D.,  for  40  in.,  +3.25  D.,  and  for  27  in.,  +3.75  D.  In  this 
case  the  band  extends  across  the  horizontal  meridian,  and 
the  straight  edge  when  seen  is  also  horizontal.  The  latter 
is  only  seen  when  we  tilt  the  mirror  vertically,  and  when 
the  reflex  comes  into  or  leaves  the  pupil.  Horizontally 
the  reflex  has  no  defined  beginning  or  end,  but  streams 
across  the  pupil  in  a  continuous  flow. 

If  we  now  use  a  cylinder  its  axis  must  be  placed  at  180, 
or  along  the  neutralized  meridian,  to  neutralize  the  other 
meridian;  and  such  cylinder  requires  to  be  minus,  for  the 
vertical  is  the  least  hyperopic  and  has  therefore  been  made, 

by  the  plus    sphere    employed,    the 
^cyuifder  most  myopic.     A  —0.75  cyl.  ax.  180 

wdll  be  found  to  be  the  cylinder  re- 
quired. But  it  will  not  be  so  easy  a  matter  to  decide  that 
it  is  just  the  value  needed,  for  a  — 0.63  cyl.  or  a  — 0.88 
cyl.  come  very  close  to  the  correction.  The  difficulty  in 
distinguishing  which  is  preferable  is  partly  due  to  the  fact 
that  either  of  the  three  cylinders  so  reduce  the  astigmatism 

61 


that  only  the  most  finely  trained  eye  can  detect  the  dif- 
ference in  motion,  for  the  banded  appearance  is  practically 
eliminated  and  serves  no  longer  as  a  guide  to  the  test.  But 
to  reduce  the  sphere  0.75  D.,  leaving  the  astigmatism  un- 
corrected, exposes  the  test  to  the  danger  of  accommodative 
resistance,  although  in  this  particular  case  that  danger  is 
avoided  by  the  low  degree  of  the  astigmatism.  In  this 
case  therefore,  by  such  spherical  reduction,  the  vertical 
meridian  v^ould  become  neutral,  and  the  band  and  straight 
edge  would  shift  from  the  horizontal  to  that  meridian.,  The 
banded  appearance  would  be  in  the  vertical  and  a  hori- 
zontal tilting  of  the  mirror  would  show  motion  and  the 
vertical  straight  edge.  If  these  results  were  set  down  we 
would  have  as  follows: 


At  53  in. 

At  40  in. 

At  27  in. 

+2.25 

+2.50 

+3 

+3 

+3.25 

+3.7 

Deduct  0.75 

Deduct  1 

Deduct  1.50 

+1.50 

+1.50 

+  1.50 

+2.25 

+2.25 

+2. 

All  of  the  findings  reduce,  after  making  the  proper  reduc- 
tion for  the  testing  space,  to  the  same  correction,  which  is 
of  course  the  prescription  first  given. 

Had  the  case  been  one  requiring  minus  in  all  meridians 
the  initial  test  before  a  lens  was  inserted  would  have  shown 
that  condition.    We  would  then  have  imposed  increasing 

62 


minus  spheres  until  one,  the  least  myopic,  meridian  was 
neutralized  and  the-  straight  edge  ap- 
AstT  matism  peared.    With  a  minus  cylinder  with 

its  axis  along  the  neutral  meridian, 
or  by  a  straight  spherical  increase  of  the  minus  value,  the 
other  meridian  would  have  been  neutralized.  The  correct- 
ing cylinder  over  the  first  found  minus  sphere  would  neu- 
tralize all  meridians  together  and  eliminate  the  banded  ap- 
pearance more  and  more  as  the  correction  was  approached ; 
but  a  spherical  change  from  one  principal  meridian  to  the 
other,  while  preserving  the  band,  would  expose  the  test  to 
the  danger  of  accommodative  action  the  same  as  in  the 
case  last  described.  The  deduction  in  the  case  of  minus 
findings  would  depend  upon  whether  the  test  was  carried 
merely  to  the  point  of  neutralization  or  to  the  reversal  of 
motion.  A  reversal  of  motion  would  of  course  show  an 
overcorrection,  and  in  that  case  the  addition  would  need 
to  be  correspondingly  reduced  to  make  up  for  the  testing 
distance.  For  a  53  in.  space,  for  instance,  the.  addition 
might  be  0.50  D.  instead  of  0.75  D.,  all  meridians  being 
treated  the  same. 

Cases  of  shadow  testing  give    frequent    examples    of 
eyes  requiring,  for  neutralization,  a  plus  value  in  one  prin- 
cipal meridian  and  a  minus  value  in  the  other.     These  are 
not  necessarily  cases  of  mixed  astigmatism,  for  the  deduc- 
tions and  additions  may  take  them  out 

Astiginatism.  ^^  ^^^*  ^^^^^'  ^^^  t^^^  ^^^  mixed  find- 

ings. If  the  eye  is  neutralized  to  give 
the  findings,  each  should  be  corrected  for  the  precise  value 
of  the  testing  space,  no  more  and  no  less.  But  if  both 
findings  are  an  overfinding  or  reversal,  the  plus  finding 
contains,  as  a  finding,  too  much  plus ;  and  the  minus  finding 
contains,  as  a  finding,  too  much  minus.  It  is  customary 
among  those  who  practice  this  method  to  deduct  the  full 

63 


space  value  from  the  plus  finding  but  to  add  a  quarter  of 
a  diopter  less  than  the  space  value  to  the  minus  finding. 
That  is,  for  a  space  of  53  in.  the  findings  and  correction 
would  be  shown  below: 


Findingfs: 

—1.25 

+0.75 


Correction: 
-1.75 
0 


That  is,  since  the  findings  themselves  tend  to  show  more 
astigmatism  than  there  is,  the  correction  is  so  modified,  and 
in  favor  of  less  minus,  as  to  negative  the  overcorrection. 


64 


CHAPTER  VIII. 

THE  SECOND  MERIDIAN. 

The  banded  appearance,  or  straight  edge  between  reflex 
and  shadow,  made  to  show  in  shadow  testing  astigmatism 
is  more  pronounced  in  the  higher  and  less  pronounced  in 
the  lower  degrees  of  that  optical  defect.  It  is  of  so  dis- 
tinctive a  character  that  its  preserva- 
^^^•^''^•.f^'''*''*  tion  throughout  the  test  is  desirable. 
Neutralizing  the  second  meridian 
with  a  minus  cylinder,  after  the  first  has  been  determined, 
corrects  the  astigmatism  and  eliminates  this  apearance; 
while  making  spherical  reductions  for  the  second  meridian 
exposes  the  test  to  the  danger  of  arousing  accommodative 
action,  as  has  been  explained.  To  avoid  both  of  these  com- 
plications involves  special  methods.  When  the  first  meridi- 
an has  been  neutralized,  supposing  it  to  be  the  horizontal 
and  that  the  other  meridian  shows  motion  requiring  minus, 
a  minus  cylinder  of  the  right  value,  axis  180,  w^ill  neutralize 
the  vertical;  but  a  minus  cylinder  of  a  higher  value  than 
the  astigmatism  will  cause  the  vertical  .focus  to  take  a  posi- 
tion back  of  the  horizontal  and  reverse  the  astigmatism,  or 
make  it  artificially  against  instead  of  with  the  rule.  If 
the  astigmatism  is  thus  overcorrected  for  more  than  the 
dioptric  equivalent  of  the  testing  space,  it  will  tend  to 
arouse  accommodative  action,  or  stimulate  the  resistance 
of  the  accommodation. 

Since  astigmatism  is  merely  the  difference  in  the  refrac- 
tion of  the  two  principal  meridians,  and  the  measure  of  it 
is  the  cylinder  that  unifies  them,  it  is  evident  that  whatever 

65 


minus  cylinder  will  neutralize  the  second  meridian  after 
the  first  has  been  neutralized,  an 
Astigmatism.  ^^^^^  P^^^  Cylinder  with  its  axis  at 
right  angles  to  the  neutralized  merid- 
ian will  also  correct  the  astigmatism,  and  that  a  higher 
plus  cylinder  similarly  placed  will  also  reverse  the  astigma- 
tism. The  latter  cylinder  would  have  its  value  or  power 
along  the  neutralized  meridian  and  destroy  its  neutrality, 
but  as  that  has  become  a  finding,  we  no  longer  care  for  ii. 
Such  a  cylinder  will  not  expose  either  meridian  to  the 
danger  of  stimulating  the  accommodation.  .Therefore, 
having  neutralized  the  first — the  most  hyperopic  or  the 
least  myopic  meridian — with  a  sphere,  we  set  the  result 
down  as  a  finding  for  that  meridian,  and  then  impose  a 
strong  plus  cylinder  with  its  axis  ^across  the  neutral  merid- 
ian over  the  sphere,  thereby  placing  it  out  of  accommo- 
dative danger,  and  then  proceed  to  reduce  the  sphere  for 
the  other  meridian.  When  it  is  neutralized  we  have  our 
second  finding.  The  cylinder  thus  imposed  must  be  suffi- 
ciently strong  to  reverse  the  astigmatism,  and  may  be  in- 
creased if  found  insufficient,  to  preserve  the  characteristic 
band  and  straight  edge.  With  the  neutralization  of  the 
vertical  in  this  manner  the  band  and  straight  edge  become 
vertical.  Care  has  of  course  to  be  exercised  in  placing  the 
axis  of  the  cylinder  exactly  at  right  angles  to  the  meridiaii) 
first  neutralized. 

An  eye  having  no  astigmatism  can  of  course  be  measured 
by  the  same  procedure,  a  plus  cylinder  creating  artificial 
astigmatism  and  a  spherical  lens  being  used  to  neutralize 
the  meridian  lying  along  the  axis  of  the  cylinder ;  then  re- 
versing the  position  of  the  cylinder, 
^^Tests*^^^  in   the   new    position    the    meridian 

under  the  axis  should  with  the  same 
sphere  be  neutral  also.     If  this  test  shows  that  a  different 


spherical  value  is  required  for  neutralizing:  the  axial  merid- 
ians astigmatism  would  be  shown,  notwithstanding  its 
nonappearance  under  a  spherical  test.  A  cylinder  may 
also  be  used  to  determine  whether  the  position  of  the  chief 
meridians  of  weak  astigmatism  has  been  correctly  deter- 
mined or  not.  If  the  position  of  the  axis  of  the  cylinder 
inserted  does  not  correspond  to  one  of  the  chief  meridians 
of  the  eye  being  tested,  the  effect  will  be  that  of  cross-cylin- 
ders with  oblique  axes,  and  the  banded  appearance  and 
movement  will  not  then  be  along  the  chief  meridians  of 
either  the  eye  or  of  the  inserted  cylinder,  but  at  some  inter- 
mediate meridian.  As  a  verification  in  a  difficmt  case  this 
method  of  corroboration  is  worth  trying. 

EMPHASIZING   THE  BAND. 

Having  discussed  the  distinctive  quality  of  the  banded 
appearance  in  shadow  testing  astigmatism,  and  stated  that 
it  is  more  pronounced  in  the  higher  degrees  of  astigmatic 
error,  there  is  another  important  fact  and  factor  in  giving 
it  the  fullest  emphasis.     The  band  is 
^iffu^sion°^  distinct  in  proportion  to  the  diffusion 

in  one  meridian  and  the  absence  of 
diffusion  in  the  other  or  opposite  meridian.  In  other  words, 
it  is  the  contrast  between  the  neutralized  and  the  un-neu- 
tralized  meridians  that  gives  the  band  its  emphatic  features. 
This  is  of  course  greater  the  higher  the  degree  of  astigma- 
tism. But  we  get  the  most  out  of  a  given  degree  of  astig- 
matism when  the  source  of  light  is  at  one  of  the  anterior 
focal  points  and  the  observing  eye  is  at  the  other.  When 
the  source  of  light  is  at  one  of  the  focal  points,  it  and  the 
retina  are,  for  that  meridian,  in  conjugate  focus.  Since 
the  other  meridian  is,  at  the  same  moment,  focused  at  the 
observing  eye,  the  latter  is  also  conjugate  to  the  retina, 
but  by  the  action  of  another  meridian.  Now,  further,  as  it 
is  the  source  of  light  that  produces  the  true  reflex,  that  re- 

67 


flex  is  most  diffuse  in  the  meridian  that  is  focused  at  the 
observing  eye  and  least  at  the  other.  Emergent  light  from 
that  reflex  will  therefore  be  slightly  diffuse  for  the  meri- 
dian conjugate  to  the  source  of  light,  and  greatly  diffuse 
for  the  meridian  conjugate  to  the  observing  eye. 

With  a  plane  mirror,  which  places' the  source  of  light  as 
far  back  of  the  mirror  as  the  light  is  in  front  of  it,  these 
respective  positions  are  attainable  only  for  ^  low  degree  of 
astigmatism:  thus,  if  the  testing  space  is  one  meter,  the 

observing  eye  is  at  the  1  D.  distance, 
^^T  •!!J!1.''"°  w^hile  the  light  is  at  most  not  more 

than  at  a  little  over  two  meters,  or 
0.50  D.,  distance,  and  this  limits  the  astigmatism  that  can 
be  reduced  to  the  positions  to  0.50  D.,  being  less  the  nearer 
the  true  light  is  to  the  mirror.  A  concave  mirror  places 
the  source  of  light  in  front  of  the  mirror,  and  its  distance 
from  the  observed  eye  can  be  varied  at  will:  (1)  by  moving 
the  light  toward  the  mirror,  causing  its  focus  (the  source 
of  light)  to  approach  the  observed  eye;  or  (2)  by  the  opera- 
tor taking  a  nearer  distance,  which  also  brings  the  mirror 
nearer  to  the  light  and  projects  the  focus  toward  the  ob- 
served eye.  If  it  is  desired  to  maintain  a  uniform  distance 
for  the  test,  the  light  may  -require  to  be  brought  in  front  of 
the  observed  eye,  and  should,  in  that  case,  be  so  screened 
by  a  chimney  as  to  prevent  direct  light  from  it  reaching  the 
eye  under  examination.  With  a  concave  mirror  of  10 
inches  focus,  the  usual  form  of  concave  mirror  employed 
in  shadow  testing,  with  the  operator  at  one  meter  from  the 
patient,  and  with  the  light  20  inches  from  the  mirror,  the 
source  of  light  is  placed  20  inches  forward  of  the  mirror 
and  therefore  20  inches  from  the  observed  eye.  In  these 
positions  if  the  observed  eye  is  1  D.  myopic  in  the  hori- 
zontal meridian  and  2  D.  myopic  in  tlie  vertical,  the  hori- 
zontal meridian  will  focus  at  the  observing  eye  and  the  ver- 
tical at  the  source  of  light.     These  positions  will  make  the 

68 


Re:u/ , 

^/^// 
^ 

i 

A 

nea/  L/g/if^    . 

^■. 

■'=^=:^::i: — 

:n 

\r~~~  — ~^ 

^' 

^^\^^ 

^^^.^ 

ffca/  ^iS^\ 

_— — — — -r====^"^ 

'^^^^^^^^Tlf5) 

J 

V— ^==-^=^1?^^      ^~x3^ 

^^J^eaf  L/'^hf" 

V--^^^^^-             ^|f^^^~— ^ 

^ 

=^ 

c/if                              skj^eaf  Liq/if'            

P/ane  anf/  Concavt^    A^/rror-s, 

^ 


banded  appearance  most  distinct  for  this  degree  of  astig- 
matism (1.00  D.). 

Whatever  degree  of  astigmatism  there  may  be  in  an  eye, 
the  imposition  of  spherical  lenses  before  it  does  not  increase 
or  diminish  the  amount,  so  whatever  the  initial  refraction 
of  the  eye,  aside  from  the  1  D.  of  astigmatism,  a  spherical 
value  may  be  found  that  will  neu- 
^°^iffht*  °^  tralize  the  meridian  of  least  refrac- 
tion, and  that  value  will  necessarily 
focalize  the  meridian  of  greatest  refraction  one  diopter 
nearer  the  observed  eye,  so  that  the  respective  positions  of 
the  two  anterior  foci  are  at  40  in.  and  20  in.,  or,  one  diopter 
apart,  the  former  position  being  that  of  the  observing  eye, 
the  latter  that  of  the  source  of  light.  Working  on  the 
basis  of  the  meridian  of  lowest  refraction  (i.  e.  the  most 
hyperopic  or  the  least  myopic)  being  the  first  meridian 
neutralized,  the  other  meridian  w^U  focus  nearer  the  ob- 
served eye,  and  enough  nearer  to  make  the  interval  be- 
tween the  foci  the  equivalent  of  the  astigmatic  value,  what- 
ever it  may  be.  If  there  is  less  than  one  diopter  of  astig- 
matism, the  source  of  light  can  be  moved  farther  from  the 
observed  eye  by  moving  the  light  farther  from  the  mirror, 
for  the  position  of  the  true  light  and  its  focal  point,  the 
source  of  light,  is  governed  by  the  principle  of  conjugate 
foci  the  same  as  a  lens.  The  conjugate  of  the  true  light, 
for  this  mirror,  cannot  however  be  brought  much  nearer 
than  13  inches,  or  27  inches  from  the  observed  eye  for  a 
one  meter  space.  To  bring  it  nearer  the  true  light  would 
have  to  be  moved  farther  than  one  meter  away,  and  a  two 
meter  distance  would  only  bring  the  conjugate  to  about  12 
inches,  and  that  would  be  impracticable.  The  amount  of 
astigmatism  represented  by  the  interval  from  27  inches 
to  40  inches  is  0.50  D.,  and  that  is  the  lowest  amount  of 
astigmatism  that  could  be  made  to  meet  the  requirements. 

71 


But  a  plane  mirror  would  enable  one  to  get  any  lower  de- 
gree, by  bringing  the  light  nearer  to  it. 

A  much  higher  degree  of  astigmatism  than  one  diopter 
could  be  managed  with  the  concave  mirror  however,  by 
bringing  the  true  light  closer  to  the  mirror.  With  the 
approach  of  the  true  light  toward  the  mirror,  the  focus 

recedes  toward  the  observed  eye  very 
*     co^ave^  Mwor.       rapidly.     Starting  from  a  position  at 

20  inches  from  the  mirror,  where  it 
focalizes  at  20  inches  from  the  observed  eye,  at  16  inches 
the  mirror  would  focus  the  light  at  27  inches,  or  13  inches 
from  the  observed  eye.  The  interval  between  this  point 
and  the  observing  eye  is  equivalent  to  2.00  D.,  and  this 
would  meet  the  requirement  for  that  amount  of  astigma- 
tism. A  position  of  the  true  light  13  inches  from  the  mir- 
ror would  focalize  it  at  the  observed  eye,  and  that  would 
be  past  the  limit  of  the  highest  degree  of  astigmatism,  or 
an  infinite  amount.  The  dioptric  value  of  the  interval 
grows  very  rapidly  from  a  20  in.  to  a  13  in.  position  of  the 
light,  for  that  represents  all  points  from  20  in.  to  0  in  front 
of  the  observed  eye.  For  a  point  or  position  nearer  than 
13  in.  the  mirror  focalizes  back  of  the  observing  eye,  or 
acts,  so  far  as  motion  is  concerned,  precisely  like  a  plane 
mirror.  The  movement  of  the  light  is  therefore  practically 
confined  to  a  space  extending  from  about  13  inches  to  one 
meter  forward  of  the  mirror. 

The  object  of  moving  the  light  nearer  and  farther  is 
not  to  determine  the  dioptric  value  of  its  position,  but  to 
bring  out  with  the  strongest  emphasis  the  banded  appear- 
ance of  the  reflex,  so  that  by  it  the  position  of  the  axis  of  a 

correcting     cylinder     is   "determined 
^Band**°  with   a   greater   degree   of   accuracy 

than  can  usually  be  obtained  subjec- 
tively.    Even  if  the  light  approaches  that  position  the  band 

72 


becomes  more  definite,  and  no  great  accuracy  in  the  location 
of  the  light  is  needed.  It  is  simply  moved  away  or  nearer, 
and  stopped  in  the  position  that  shows  the  clearest  banded 
appearance  across  the  pupil.  Taking  the  meridians  succes- 
sively, as  previously  described,  each  confirms  the  other,  for 
the  two  bands  are  necessarily  at  right  angles  to  each  other. 


73 


CHAPTER  IX. 

DIFFICULT   CASES. 

There  are  several  causes  operating  to  defeat  the  best 
endeavors  of  the  refractionist  to  shadow  test  particular 
cases.    While  a  majority  of  people  are  readily  and  success- 
fully tested  in  this  manner,  an  outline  of  the  difficulties 
that  interfere  in  some  cases  will  not 

er  people,  those  whose  statements 
cannot  be  relied  upon  in  a  subjective  examination  or  who 
are  too  young  to  answer  intelligently,  are  most  easily  shad- 
ow tested.  The  reflex  is  vivid  and  its  motion  is  readily 
determined.  In  older  people  there  is  more  or  less  cloudi- 
ness and  lack  of  brilliancy  and  also  a  small  and  fixed  pupil- 
lary area.  But  the  rule  is  only  a  general  one,  some  old  peo- 
ple being  very  easily  shadow  tested  while  some  young  ones 
are  exceedingly  difficult.  Light  complexioned  people  are 
as  a  rule  more  easily  tested  than  dark  ones,  for  the^  fundus 
partakes  in  color  tone  of  the  same  general  character  as  the 
complexion, — is  light  in  the  light  people  and  dark  in  the 
dark  ones.  This  is  no  doubt  due  to  the  saturation  of  pig- 
ment in  the  choroid  immediately  back  of  the  retina,  the 
darker  colors  more  fully  absorbing  the  light  and  giving  a 
less  brilliant  reflex.  Negroes  are  not  easily  shadow  tested 
as  a  rule. 

Attention  has  been  called  to  the  tendency  of  the  eye, 
under  the  influence  of  the  brilliant  light  that  is  cast  upon 
it  in  shadow  testing,  to  accommodate  and  thus  give  a  find- 

74 


ing  not  in  accordance  with  the  real  static  refraction.  But 
an  eye  that  does  not  at  first  give  a 
Sensitiveness.  satisfactory  reflex  is  not  therefore  to 
be  regarded  as  being  a  ''hard  case,'* 
for  the  imposition  of  a  plus  lens,  relaxing  the  accommo- 
dative effort,  often  causes  the  reflex  to  at  once  show  up 
and  the  eye  to  become  entirely  submissive  to  the  test,  not- 
withstanding the  fa.ct  that  it  at  first  gave  back  no  satis- 
factory motion.  But  a  too  brilliant  light — one  that  daz- 
zles the  eye — is  not  the  best  sort  to  employ.  Cases  with  real 
photophobia,  inability  to  submit  to  the  test  because  of 
supersensitiveness  of  the  retina,  cannot  be  tested  satisfac- 
torily by  the  method.  The  light  causes  a  general  nervous 
agitation  that  no  power  of  the  will  can  overcome,  and  a 
shadow  test  to  be  reliable  requires,  above  everything  else, 
a  state  of  complete  passivity. 

BINOCULAR  INFLUENCE. 

Beside  the  difficulties  enumerated  above  there  is  another 
class  of  complications  that  arises  from  the  binocular  rela- 
tionship of  the  eyes,  or  their  habit  of  acting,  not  as  single 
organs,  but  as  a  team  working  together.     It  is  known  that 

the  two  eyes  under  the  usual  circum- 
^^'^^Eye^^^  stances   of   vision   accommodate   and 

relax  together,  and  for  equal 
amounts ;  and  furthermore  it  is  the  active  eye  that  controls, 
not  the  passive.  If  the  two  eyes  are  in  such  a  refractive 
condition  that  one  is  compelled  to  -accommodate  whilq  the 
other  need  not,  the  eye  requiring  to  accommodate  will  do 
so  and  the  other  eye  will  be  compelled  to  accommodate 
equally  at  the  same  time.  For  eyes  habituated  to  such 
binocular  harmony  of  accommodative  action,  Ho  artificial 
means  of  putting  them  in  a  different  relationship  will  be 
effective  in  suppressing  the  habit.  Hence,  in  shadow  test- 
ing a  case,  to  test  one  eye  while  the  other  is  uncovered  ex- 

75 


poses  the  test  to  the  binocular  influence  tending  to  give  a 
false  finding  for  the  eye  tested.  The  accommodation  re- 
quires to  be  relaxed  in  both  eyes  to  be  relaxed  in  either. 
This  danger  arises  in  hyperopic  cases  only,  but  that  in- 
cludes a  large  percentage  of  all  cases. 

In  isometropia  (equal  refractiQji  or  refractive  error) 
this  difficulty  must  be  specially  guarded  against,  for  the 
habit  of  accommodative  unison  is  strong,  indissoluble.  In 
anisometropia  (unequal 'refraction  or  refractive  error)  the 

habit  is   not  so   indissoluble,   but  it 
metropia.  Cannot  be  ignored  in  any  kind  of  a 

case.  In  shadow  testing  a  pair  of 
eyes  we  are  not  of  course  informed  to  begin  with  in  what 
state  the  eyes  are  in  this  respect.  It  is  feasible,  however, 
to  treat  them  as  equals  until  their  true  relationship  is  as- 
certained. This  is  done  by  imposing  equal  lenses  before 
them  until  the  motion  of  the  shadow  is  reversed  in  both 
eyes.  They  may  reverse  together  or  one  may  require  a 
higher  lens  than  the  other.  By  selecting  the  eye  requiring 
the  higher  plus  lens  as  the  one  first  to  be  neutralized  or 
reversed,  leaving  the  other  eye  in  the  meantime  in  the  fog; 
and  then,  with  the  neutralizing  lens  on  the  first  eye,  simi- 
larly neutralizing  the  other,  gets  us  over  this  difficulty. 
We  will  find  out  by  the  test  whether  the  eyes  are  equal  or 
not,  but  while  doing  so  have  eliminated  the  binocular  uni- 
son of  accommodative  action  as  a  disturbing  element  in  the 
test. 

In  cases  of  astigmatism  the  meridian  requiring  the  great- 
est amount  of  plus,  as  well  as  the  eye  requiring  the  great- 
est amount,  have  both  to  be  looked  to,  for  astigmatism  is 
nothing  more  than  anisometropia  of  different  meridians 

of  the   same   eye.     To   eliminate   all 
Binocular  tendencies    to    accommodation,     the 

Seiaxatiou.  ' 

more  hyperopic  meridian  of  each 
must  be  ' '  plussed '  *  out  of  that  tendency,  not  only  for  that 

76 


eye  in  particular  but  for  its  influence  on  the  accommoda- 
tion of  the  other  eye.  The  rule  therefore  should  be  "Get 
myopic  motion  in  all  meridians  of  both  eyes  before  attempt- 
ing to  neutralize,  by  reduction,  the  least  myopic  meridian 
of  either."  Having  fogged  both  eyes  under  this  rule,  one 
is  left  so  covered,  the  one  requiring  the  most  plus  of  the 
two,  and  the  first  meridian  of  the  other  eye  is  then  neutral- 
ized by  reducing  the  sphere.  The  procedure  from  this 
point  has  already  been  described  under  astigmatism.  With 
the  neutralizing  lens  for  the  first  eye  before  it,  the  other  eye 
is  then  treated  in  the  same  manner.  This  method  is  better 
than  merely  covering  one  of  the  eyes  with  an  opaque  disc 
while  the  other  is  being  tested,  for  it  insures  the  co-opera- 
tion of  the  two  eyes  under  the  test,  as  they  have  been  ac- 
customed to  co-operate.  , 

MUSCULAR  POISE. 

But  there  is  still  another  binocular  influence  to  be  reck- 
oned with  in  shadow  testing  the  eyes.  The  connection  be- 
tween the  reflexes  by  which  the  eyes  accommodate  and  con- 
verge, or  relax  those  functions  together,  is  well  understood. 

If  there  is  a  normal  poise  of  the  extra- 
*~ctret^enc"."*     °<'^^^^  i»"scles,   no   abnormal   influ- 

ence  is  exercised  by  them  over  the 
accommodation;  but  in  certain  muscular  imbalances,  par- 
ticularly that  imbalance  that  is  known  as  exophoria,  the 
influence  may  be  considerable.,  That  is,  exophoria,  since 
it  is  a  tendency  of  the  eyes  to  turn  outward,  naturally 
arouses  counteraction  by  the  internal  recti  muscles.  The 
tendency  to  diplopia  that  the  outward  turning  of  the  eyes, 
or  tendency  so.  to  turn,  awakens  by  reflex  action  the  coun- 
ter stimulation  of  the  internal  muscles.  As  these  muscles 
are  innervated  by  the  same  pair  of  cranial  nerves  that  con- 
trol ciliary  activity,  the  ciliary  is  involved,  or  apt  to  be 

77 


involved,  and  that  results  in  accommodation,  even  though 
there  is  no  direct  cause,  aside  from  such  influence,  to  pro- 
duce it.  To  relax  the  accommodation  of  an  eye,  it  is 
therefore  not  only  necessary  to  give  that  eye  no  cause  for 
action,  but  the  accommodation  of  the  other  eye  must  also 
be  suppressed,  and  now  a  pair  of  muscles  entirely  outside 
of  the  eyes  must  be  made  inactive  also.  This  influence  can 
be  overcome  by  excluding  one  of  the  eyes  from  vision — 
that  is,  by  covering  it  with  an  opaque  disc.  But  it  can 
also  be  overcome,  for  the  time  being,  by  inserting  over 
either  eye  a  prism,  base  in,  of  sufficient  power  to  relax  the 
internal  muscles,  or  to  allow  the  eyes  to  turn  outward  ac- 
cording to  their  tendency  without  producing  diplopia. 

Lest  the  complications  here  described-  discourage    the 
would  be  shadow  test  expert,  let  me  say  that  such  compli- 
cations are  infrequent,  and  they  are  brought  up  here  for 
the  purpose  of  giving  the  student  of  shadow  testing  an 
understanding  of  some  of  the  cases 
Special  ^YiSi^  might  otherwise  defeat  his  best 

Metnoas. 

efforts,  and  cause  him  to  forswear 
the  use  of  the  method  under  any  and  all  circumstances.  As 
to  the  method  that  has  been  christened  by  its  inventor  ''dy- 
namic skiascopy  with  a  retino-skiameter"  I  would  say  that 
while  I  am  sure  that  that  erudite  and  euphonious  phrase 
ought  to  be  sufficient  to  paralyze  the  most  obstinate  ciliary 
spasm  that  ever  came  out  of  the  woods,  I  have  only  been 
able  to  verify  the  fact  that  it  has  the  effect  of  paralyzing 
the  tongue  of  one  who  tries  to  repeat  it  verbatim.  And 
as  to  the  use  of  atropine,  or  any  other  drug,  as  an  aid  in 
determining  the  refractive  condition  of  the  eye,  either  by 
the  shadow  test  or  any  other  method,  about  all  the  impor- 
tance that  attaches  to  it  is  that  ''it  is  a  convenient  means 
of  discovering,  while  the  patient  is  alive,  what  kind  of 
lenses  will  be  adapted  to  his  eyes  after  he  is  dead."     Or, 

78 


e 

^^^^H 

I 

ZOA 

1 

1 

Ex  centric    T^effex 

1                     1 

1 

■■ 

la 

m 

£:xct 

'/1/r/c  ne^ 

J  ex 

<JciSsors  A^oye/nent' 

1 

T^ecu/ia*^ 

Ref/ex    Effects 

the  proposition  may  be  stated  in  this  manner:  "It  is  a 
method  of  concealing  the  things  you  most  wish  to  know 
about  the  eyes  for  the  purpose  of  discovering  what  you 
don't  care  anything  about,  and  which  is  of  no  importance 
to  anyone."  One  might  as  well  go  to  the  morgue  after  his 
cases  as  to  go  to  atropine  to  fit  them. 

There  is  an  effect  not  described  under  any  of  the  above 
statements  that  often  rises  to  confuse  the  beginner.  That 
is,  as  neutralization  is  approached,  the  reflex  shows  a  ten- 
dency to  split  up  in  various  ways  and  follow  no  consistent 

direction  of  motion,  part  of  it  going 
Zonular  ^  direction  and  another  part  in 

the  opposite  direction,  or  areas  near 
the  margin  of  the  pupil  showing  motion  toward  the  cen- 
ter while  areas  at  the  center  move  toward  the  margin. 
This  is  especially  apt  to  result  when  mydriatics  are  used. 
In  astigmatic  cases  these  peculiarities  are  combined  with 
a  partially  banded  appearance  that  gives  the  whole  the 
effect  of  the  coming  together  or  separation  of  the  blades  of  a 
pair  of  scissors.  These  effects  do  not  become  manifest  ex- 
cept at  the  near  approach  of  neutralization,  and  they  are 
an  indication  that  the  eye  is  all  but  neutralized.  They 
are  supposed  to  be  due  to  slight  spherical  aberration  of  the 
dioptric  media,  such  that  when  one  area  of  the  pupil  is 
neutral  other  areas  of  a  slightly  greater  or  less  refractive 
power  are  still  in  a  non-neutral  condition  and  contradictory 
effects  are  the  result.  The  effect  illustrates  the  exactness 
of  the  shadow  test  method,  where  exact  neutralization  can 
be  effected  only  by  establishing  the  point  of  reversal  ex- 
actly at  the  nodal  point  of  the  observing  eye. 


81 


CHAPTER  X. 

SHADOW  TEST  DEVICES. 

The  shadow  test  method  of  fitting  the  eyes  requires  the 
employment  of  special  devices  made  for  that  purpose.     The 
mechanical  structure  of  these  devices  varies  greatly,  giving 
the  refractionist  a  number  to  choose  from.     Where  an  elec- 
tric current  is  provided,  electric  light- 
chimneys!  ^^^  ^^  *^®  ^^^^  Satisfactory  and  con- 

venient for  all  optical  purposes.  For 
shadow  testing  the  round  frosted  electric  bulb  is  prefer- 
able, as  in  it  the  wires  are  to  an  extent  concealed,  the 
radiance  of  the  entire  bulb  furnishing  the  light.  It  is 
necessary  only  to  have  the  light  so  situated  that  its  radiance 
is  permitted  to  fall  upon  the  mirror  but  not  on  the  eye  or 
face  of  the  patient,  and  with  an  unscreened  light  this  posi- 
tion is  necessarily  back  of  the  patient's  head.  However, 
this  does  not  permit  locating  the  light  for  developing  the 
banded  appearance  in  astigmatism,  as  described  in  a  former 
chapter.  To  permit  such  movement,  the  light  must  be  so 
screened  as  to  allow  its  radiance  to  take  but  one  course, 
and  that  in  the  direction  of  the  mirror.  The  screen  so 
uesd  is  known  as  the  skiascopic  chimney,  and  it  is  made 
usually  of  asbestos  or  metal.  It  is  so  shaped  as  to  fit  over 
the  lamp  or  flame  and  screen  the  light  everywhere  except 
at  an  opening  opposite  the  light,  which  is  turned  toward 
the  mirror. 

There  are  a  number  of  •  forms  of  these  chimneys,  some 
with  revolving .  discs  that  give  a  particular  form  to  the 
light,  as  round,  oblong,  lineal,  a  cross,  etc  Another  form 
is  provided  with  an  iris  diaphragm,  so  that  the  light,  which 

82 


is  always  round  in  form,  may  be  made 
Li^ht  large  or  small.     With  a  chimney  of 

Brackets.  ^  *^ 

this  kind  screening  the  light,  it  may 
be  brought  forward  of  the  eye  being  examined  without  its 
radiance  falling  on  the  patient's  eye,  and  thus  provide  the 
facilities  for  bringing  out  the  banded  appearance  in  astig- 
matism. But  to  be  movable  in  this  manner  the  light  must 
be  attached  to  a  bracket  of  some  kind,  permitting  the  re- 
quired range  of  movements.  A  variety  of  these  brackets, 
some  for  gas  and  some  for  electric  lights,  are  therefore 
among  the  devices.  All  of  the  forms  of  the  light  bracket 
permit  a  degree  of  motion  to  either  side,  but  they  also  allow 
the  light  to  be  drawn  toward  the  operator  and  mirror, 
which  is  their  chief  convenience  as  shadow  test  devices. 
The  chief  objection  to  gas  as  a  shadow  test  light  is  the  heat 
of  it,  both  to  the  patient  and  the  operator,  but  there  are 
compensating  advantages  in  the  uniformity  of  the  light 
from  the  flame,  and  gas  is  used  almost  as  much  as  electricity 
^for  this  purpose.  For  the  ophthalmoscope  a  gas  light  is 
better  than  electric  light,  because  of  its  greater  brilliancy, 
and  volume,  and  as  the  bracket  light  arrangement  is  for 
the  double  purpose  of  shadow  testing  and  ophthalmoscopy 
usually,  this  in  part  accounts  for  its  employment.  An 
argan  kerosene  burner  is  also  much  used  in  the  smaller 
towns,  as  well  as  acetylene  gas. 

The  mirror  used  in  shadow  testing  is  of  diverse  forms 
and  sizes,  a  matter  of  preference  purely.  The  concave  mir- 
ror is  i^sed  by  many,  but  the  plane  mirror  is  more  gener- 
ally preferred.     As  to  size,  the  beginner  will  find  a  larger 

mirror   preferable,   not   that   it   pro- 
Btorors.^  vides  any  more  light  to  the  eye,  but 

it  gives  a  wider  field  of  illumination 
and  is  more  easily  directed  into  the  eye  on  that  account. 
The  small  mirror  is  a  rifle  requiring  a  steady  aim,  while  the 

85 


larger  mirror  is  a  shot  gun,  easily  directed  to  the  right 
point.  In  any  sort  of  a  plane  mirror  a  good  deal  of  the 
light  is  misdirected  and  wasted.  This  has  led  the  writer  to 
devise  a  mirror,  or  rather  a  lens-mirror,  that  has  a  suffi- 
cient plus  value  to  neutralize  the  distance  of  the  light  from 
it,  and  intensify  the  light  entering  the  eye,  but  giving  the 
same  character  of  motion  as  the  plane  mirror.  Held  at 
one  meter  from  the  light  it  throws  a  brilliant  circle  of  light, 
the  same  size  as  the  mirror,  on  the  face,  which  is  easily 
directed  into  the  eye  being  examined  and  which  gives  a 
brilliant  reflex,  even  for  eyes  that  ordinarily  do  not  re- 
spond to  the  shadow  test  in  a  satisfactory  manner. 

To  become  skillful  in  shadow  testing  requires  practice 
and  persistence.  It  is  too  much  of  an  imposition  to  ask 
a  friend  or  member  of  the  family  to  pose  for  you  while  you 
are  getting  acquainted  with  the  primary  phenomena.  To 
avoid  this  the  schematic  eye  is  used. 
^°^ye****'  This  is  a(;  paper  or  metal  affair  made 

up  in  imitation  of  the  eye.  It  of 
course  has  no  accommodation,  but  can  be  set  at  any  desired 
degree  of  hyperopia  or  myopia  by  the  sliding  tube  arrange- 
ment of  the  body.  The  posterior  surface  is  lithographed  in 
imitation  of  the  human  retina,  so  that  the  appearance  of 
the  reflex  is  not  unlike  the  retina  itself.  In  front  there 
are  cells  to  contain  whatever  lenses  you  may  desire  to  im- 
pose before  it.  The  author  has  devised  such  a  paper  sche- 
matic eye  with  auxiliary  cylindrical  discs  that  introduce 
astigmatism,  and  give  the  beginner  a  general  idea  of  the 
appearances  of  the  reflex  in  testing  astigmatism.  The 
cylindrical  discs  do  not  interfere  with  the  insertion  of  lens 
corrections  in  the  cells  before  the  eye.  The  practice  work 
with  a  schematic  eye  is  more  fully  set  forth  in  a  subsequent 
chapter. 

The  medical  refractionist   is  taught  that  mydriasis  is 


necessary  to  successful  shadow  test  work,  and  as  a  rule  he 
employs   atropine   or   other  drug   for  the   purpose.     But 
apparently  in  denial  of  this  doctrine,  after  dilating  the 
pupil  an  opaque  disc  with  a  smaller 
^1   ^^  .pupillary  opening  is  put  before  the 

eye,  the  object  is  to  shut  out  the  very 
areas  that  have  been  exposed  by  the  dilatation.  Such  a 
disc  is  a  good  thing  to  use,  if  atropine  is  employed,  for  it 
narrows  the  tested  area  to  that  that  is  employed  for  visual 
purposes.  If  the  drug  is  also  cycloplegic  in  effect,  a  re- 
laxed accommodation  and  the  visual  area  of  the  pupil  are 
both  obtained.  But  by  this  method,  if  thorough  cyclopile- 
gia  is  produced,  more  plus  is  found  than  the  patient  can 
wear,  and  a  part  is  always  deducted.  The  uncertainty  of 
the  method  is  then  in  the  uncertainty  of  the  deduction.  But 
the  pupillary  disc  with  a  white  arrow  across  it,  to  show  the 
positions  of  the  chief  meridians  in  cases  of  astigmatism,  is 
a  proper  device.  It  may  be  satisfactorily  employed  without 
the  use  of  a  mydriatic. 

More  pretentious  than  any  of  the  devices  thus  far  de- 
scribed is  the  luminous  retinoscope.     This  contains  a  small 
incandescent  lamp,  which  is  lighted  by  a  dry  battery,  in 
a  tube  so  placed  before  an  inclined  mirror  that  the  light 
is  reflected .  from  the  mirror  at  right 
B^tino'^c^pe.         angles  to  the  handle,   or  in  the  di- 
rection of  the  eye  to  be  observed.    The 
best  feature  of  this  device  is,  that  it  is  only  necessary  to 
keep  in  view  one  objective  point,  the  eye  to  be  examined. 
The  position  of  the  light  remains  unchanged,  however  the 
mirror  may  be  tilted,  and  to  "shoot"  it  to  the  eye  to  be 
examined  is  a  single  proposition.     Such  a  light  has  also, 
because  of  a  condensing  lens  between  it  and  the  mirror, 
superior  brilliancy,  and  that  may  be  increased  and  reduced 


as  desired  by  governing  the  number  of  battery  cells  en- 
gaged, which  is  regulated  by  the  battery  used. 

In  addition  to  the  devices  described  above  there  is  a  vari- 
ety of  devices  serving  to  place  the  lenses  in  front  of  an  eye 
being  shadow  tested.     They  consist  of  some  metal  or  wooden 
form  with  a  handle  or  a  revolving  disc,  to  be  held  by  the 
patient  so  as  to  place  a  lens  before 
sc^^pes  *^^  ^y^'    ^  device  of  this  kind  has 

been  called  a  skiascope.  The  rotating 
disc  containing  a  battery  of  plus  and  minus  spheres  is  un- 
doubtedly the  best  of  these  forms.  It  may  be  mounted  on  a 
standard.  The  same  device  is  also  employed  in  subjective 
testing  as  a  substitute  for  the  trial  frames,  and  is  not  there- 
fore strictly  a  shadow  test  device. 


88 


CHAPTER  XI. 

SHADOW  TEST  INSTRUMENTS. 

In  a  method  so  interesting,  and  to  the  extent  that  it  is 
known  and  practiced,  popular,  it  is  natural  to  expect  the 
embodiment  of  its  essential  principles  in  instruments  that 
are  conceived  to  render  its  practice  more  easy  and  certain. 

There  have,  however,  been  but  two 
instTOm^nts  attempts  to  improve  upon  the  natural 

or  open  method.  The  tw^o  instru- 
ments referred  to  are  the  Geneva  Retinoscope  and  the 
Cross  Retino-Skiameter.  The  former  was  first  put  forth 
as  the  Prentice  Retinoscope  and  suggested  by  Dr.  Chalmers 
Prentice.  There  was  nothing  "inventive"  about  it  save 
its  mechanical  structure,  for  it  was  the  open  method  plus  a 
brass  tube  and  a  battery  of  lenses  in  a  rotary  disc.  The 
mirror  and  light  were  of  course  placed  so  as  to  facilitate 
throwing  reflected  light  into  the  eye  to  be  examined,  and  a 
place  is  provided  to  hold  the  eye  at  the  other  end  of  the 
tube.  In  the  later  instrument  the  tube  is  eliminated  and 
the  eye  is  shadow  tested  across  an  open  space,  the  same  as 
in  the  open  metliod. 

Mechanically  considered  such  an  instrument  facilitates 
easy  shadow  testing,  for  it  does  away  with  trial  frames, 
space  measurements,  deductions,  etc.  The  lens  that  neu- 
tralizes the  eye,  or  one  of  its  meridians,  is  the  full  finding 

without   further   modification.      This 
Be«^osIope.         result  is  obtained  by  so  modifying  the 

notation  of  the  lenses  that  the  re- 
quired deduction  is  a  part  or  factor  in  the  finding.  The 
distance  of  the  test  is  necessarily  the  same,  for  the  instru- 

89 


ment  sets  the  observed  and  the  observing  eyes  a  fixed  dis- 
tance apart.  The  mirror  is  also  so  adjusted  in  position 
that  Jio  aiming  is  necessary.  The  light  falls  directly  and 
immediately  upon  the  eye  when  the  latter  is  placed  in  tlie 
proper  position  and  the  light  is  turned  on.  FVom  this 
position  it  can,  however,'  be  tilted  in  either  direction  at 
right  angles  to  the  handle,  sets  of  opposite  springs  return- 
ing it  to  its  regular  position.  The  lateral  rotation  of  the 
mirror  allows  tilting  the  mirror  in  any  meridian,  while 
a  pointer  and  half  circle  of  degree  marks  locate  exactly 
the  meridian  tested. 

The  lenses  that  are  imposed  before  the  eye  are  contained 
in  two  circular  discs,  one  disc  containing  a  double  battery 
of  plus  and  minus  spheres  varying  one  diopter,  from  — 8 
D.  to  -f-7  D.     Another  circular  disc  contains  the  fractional 
auxiliary  lenses,    from    — .75  D.  to 
reaTures!  +.75  D.,  SO  that,  by  the  combination 

any  'desired  value,  integral  or  frac- 
tional, can  be  obtained.  Every  such  combination  contains 
the  deduction  for  the  distance  of  the  test,  so  the  finding  is 
correct  as  found.  For  a  higher  degree  of  plus  or  minus 
than  the  discs  provide  for  an  auxiliary  cell  for  the  inser- 
tion of  a  lens  from  the  trial  case  is  provided.  This  cell 
may  also  be  used  for  the  imposition  of  a  cylinder  after  the 
manner  described  in  a  former  chapter  for  the  purpose  of 
neutralizing  any  tendency  to  accommodate  when  testing 
for  the  second  or  less  hyperopic  meridian,  if  that  is  foiindj 
in  a  particular  case,  to  be  necessary.  In  the  rotation  of 
these  discs  before  the  eye  during  a  test  there  is  no  disturb- 
ance of  the  distance  between  the  operator  and  patient,  nor 
is  there  any  unhandiness  in  working  out  a  finding  in  the 
least  time  and  at  as  little  inconvenience  to  the  operator  and 
patient  as  possible.  As  heretofore  stated,  these  advan- 
tages are  purely  mechanical. 

90 


The  ** binocular  influence'"  referred  to  in  a  previous 
chapter  is  also  met  by  a  special  cell  to  contain  a  fogging 
lens  before  the  eye  that  is  not  being  tested  at  the  time. 
This  cell  and  the  fogging  lens  is  so  hinged  to  the  instrument 
that   it   is   readily  swung,    into    the 
^ceus*^  proper  position  to  cover  the  other- 

wise uncovered  eye  and  is  an  impor- 
tant factor  to  the  test,  showing  the  intimate  relation  that 
exists  between  the  accommodation  of  the  two  eyes.  The  cell 
is  in  duplicate,  so  that  whichever  eye  is  being  tested  the 
other  can  be  fogged.  This  device  or  auxiliary  is  also  purely 
mechanical..  Take  the  purely  mechanical  features  out  of 
the  Geneva  Retinoscope  and  you  have  nothing  left  but  the 
open  method  of  shadow  testing.  We  are  not  therefore 
inclined  to  give  credit  for  inventive  genius  to  anyone  con- 
nected with  devising  the  instrument,  except  only  the  one 
or  many  who  worked  out  a  familiar  practical  principle  in 
a  correct  mechanical  manner.  In  other  words,  the  virtues 
of  the  instrument,  over  and  above  the  open  method  of 
shadow  testing,  are  mechanical  and  nothing  else.  But 
these  mechanical  points  are  exceedingly  valuable,  time  sav- 
ing, and  contributory  to  exactness.  The  one  little  me- 
chanical point  of  inclining  the  lenses  so  as  to  avoid  reflec- 
tions has  more  virtue  as  a  "discovery"  or  "invention" 
in  it  than  the  whole  concept  of  the  instrument  as  an  en- 
tirety. 

The  instrument  embodies,  however,  another  feature  that 

greatly  enhances  its  value — that  is,  its  feature  as  an  indirect 

ophthalmoscope.     This  feature,  however,  entirely  changes 

its  aspect  as  an  instrument  and  takes  it  out  of  the  shadow 

test  class.     In  the  latter  instrument 

Ophthalmoscope.       t^^re  is  more  invention  and  discovery 

and  less  mechanical  art.      But    the 

discussion  of  this  feature  is  not  relevant  in  a  book  on  the 

shadow  test. 

93 


In  the  Cross  Retino-Skiameter  the  same  principles  of 
conjugate  foci  are  operative  as  in  open  shadow  testing, 
but  there  is  a  principle  of  mobility  introduced  into  the 
lens  value  that  is  used  to  neutralize  motion  not  found  in 

any  other  instrument.     The  Skiame- 
Retino^skiLeter.      ^^^  consists  of  two  metal  tubcs,  one 

for  each  eye,  with  a  stationary  minus 
value  next  to  the  eye,  and  before  this  is  a  mobile  plus  lens 
— a  lens  that  may  be  drawn  forward.  Originally  the  two 
values,  both  plus  and  minus,  were  cross  cylinders  so  in- 
clined along  the  axial  meridian  as  to  avoid  reflections  and 
not  change  their  optical  value.  Whether  this  mode  of 
construction  is  still  followed  I  do  not  know.  The  forward 
plus  cylinders  were  moveable  forward  together,  or  either 
could  be  moved  singly.  If  moved  together  the  dioptric 
v^lue  of  the  movement  was  spherical,  but  if  the  forward 
of  the  two  plus  cylinders  alone  was  moved  forward,  that 
cylinder  alone  enhanced  by  the  movement. 

Light  emerging  from  the  eye  through  a  scheme  of  minus 
and  plus  lenses  of  equal  value  would  not,  if  the  lenses  were 
together,  be  materially  altered,  for  one  lens  would  neutral- 
ize the  action  of  the  other  the  same  as  one  surface  of  a  plane 

lens   neutralizes   the   action    of    the 
'^\^T^^  other,  although  both  act.     But  if  the 

forward  plus  lens  were  stronger  than 
the  minus  lens  preceding  it,  or  if  it  were  somewhat  further 
forward  than  the  minus  lens,  it  would  tend  to  focalize 
emergent  light.  Moreover,  the  focalizing  power  of  the  for- 
ward plus  lens  increases  rapidly  the  farther  it  is  removed 
from  its  antagonist.  This  is  due  to  the  fact  that  the  plus  lens 
action  is  in  harmony  with  the  natural  evolution  of  light 
waves  while  the  minus  lens  action  is  antagonistic  thereto. 
Light  waves  emerging  from  an  emmetropic  eye  in  a  state  of 
accommodative  rest  are  plane.    If  passed  through  a  — 7  D. 

94 


ry 


lens  at  that  point,  they  are  converted  into  -{-Ic  or  diverging 
waves.  A  +7  D.  lens  immediately  posterior  to  the  first 
lens  would  neutralize  them  immediately.  But,  if  the  -\-7c 
waves  are  allowed  to  evolve  naturally  for  a  space,  their 
curvature  reduces  rapidly  to  +6c,  +5c,  +4c,  -|-2c,  etc., 
requiring  constantly  less  action  .to  neutralize  them  again. 
The  farther  the  mobile  plus  lens  is  from  the  minus  the  less 
the  curvature  of  the  waves  coming  to  it,  and  therefore 
the  less  neutralizing  work  is  required  of  it,  and  therefore 
the  increase  of  its  focalizing  power. 

If  the  mobile  plus  lens,  in  such  a  scheme  of  lenses,  is 
sufficiently  stronger  than  the  minus,  or  if  the  mobile  plus 
lens  is  sufficiently  distant  from  its  antagonistic  minus  lens, 
the  system  may  be  made  to  focus  emergent  light  from  an 
emmetropic  eye  at  one  meter,  giving 
^of^Lens!°^  ^^^^  *^  ^^^  phenomena  of  neutraliza- 
tion we  have  described.  A  forward 
movement  of  the  mobile  lens  from  that  position  would  focus 
the  system  within  the  meter  space.  For  the  same  reason 
a  more  forward  position  of  the  mobile  lens,  in  hyperopia, 
would  be  required  to  (1)  relax  the  accommodation,  and 
(2)  focus  the  relaxed  eye  at  the  fixed  point,  or  one  meter. 
The  position  of  the  mobile  lens  required  to  neutralize 
motion  at  the  fixed  point  becomes  then  a  measure  of  the 
hyperopia.  In  the  construction  and  adjustment  of  the 
instrument  the  system  of  lenses  is  so  related,  in  value  and 
position,  that  emmetropia  is  neutralized  at  one  meter's  dis- 
tance. The  different  degrees  of  hyperopia  then  require  a 
different  amount  of  forward  movement  of  the  mobile  lens 
to  focalize  the  entire  system,  including  that  of  the  eye,  at 
the  position  of  observation.  In  myopic  cases  an  auxiliary 
minus  lens  is  added  to  the  fixed  minus  lens  of  the  instru- 
ment, and  the  artificial  hyperopia  introduced  is  then  meas- 
ured and  offset  against  the  auxiliary  lens  required  in  the 

97 


particular  instance.  In  this  manner  the  instrument  fully 
obeys  the  principles  of  conjugate  foci  upon  which  all 
shadow  testing  is  founded. 

The  introduction  of  the  scheme  of  lenses  employed  in  this 
instrument  develops  another  phenomenon  not  strictly  an 
integral  part  of  shadow  testing.  That  is,  the  observer  gets 
a  view  of  the  pupil  through  a  series  of  positive  and  nega- 
tive lenses  arranged  in  tandem  and 
^^t^vLvn^^  separated  by  more  or  less  space.  This 
gives  rise  to  the  phenomenon  of 
magnification,  and  the  pupil  of  the  eye  examined,  instead 
of  appearing  of  the  normal  size,  is  considerably  enlarged. 
The  enlargement  is  strictly  a  question  of  the  relative  posi- 
tions of  the  lenses:  the  farther  out  the  mobile  lens  'the 
greater  the  enlargement.  By  increasing  the  minus  lens  be- 
fore the  eye  and  augmenting  the  hyperopia,  the  farther 
forward  the  mobile  lens  is  required  to  move  to  neutralize 
motion,  so  that  any  desired  enlargement  can  be  obtained. 
But  too  great  an  enlargement  of  the  pupil  tends  to  dissi- 
pate the  reflex,  and  therefore  a  moderate  magnification  is 
preferable.  In  astigmatic  cases  it  is  possible  to  so  locate 
the  mobile  cylinders  as  to  neutralize  both  meridians,  but 
this  would  naturally  distort  the  pupil,  giving  it  an  oblong 
or  elliptical  form.  I  doubt  if  the  instrument  is  now  used 
in  this  manner. 

The  Skiameter  involves  the  consideration  of  more  com- 
plex optical  factors  and  values  than  any  other  shadow 
test  instrument  and  represents,  I  should  say,  a  higher  de- 
gree of  inventive  genius.    It  is  perhaps  on  that  account  in 
connection  with  the  rather  low  state 
Principles.  ^^  scientific  knowledge  possessed  by 

the  average  refractionist,  that  makes 
it  an  instrument  for  the  optician  of  thorough  technical 
knowledge  and  skill.     In  regard  to  the  dynamic  method, 

98 


which  may  be  used  with  or  without  the  instrument  and  of 
which  I  have  already  expressed  a  negative  opinion,  Mr. 
Cross,  in  a  recent  letter,  says:  "I  feel  that  I  have  made 
great  strides  since  I  talked  with  you  last  and  I  think  more 
of  my  dynamic  method  than  ever."  The  confidence  of  its 
inventor  or  discoverer  in  the  dynamic  method  almost  leads 
one  to  belief,  and  the  writer  confesses  to  knowing  very 
little  about  it,  and  he  possesses  also  a  very  limited  knowl- 
edge of  the  instrument,  other  than  an  abstract  proposition, 
never  having  enjoyed  the  privilege  of  a  more  concrete 
study  of  it. 

The  firm  of  F.  A.  Hardy  &  Co.,  of  Chicago,  has  recently 
put  on  the  market  an  instrument  that  is  designated  as  the 
Stigmatometer.     It  is  not  a  shadow  test  instrument,  but 
employs  factors  and  contains  features  resembling  instru- 
ments of  that  kind.     That  is,  it  is 
The  stifif-  operated  bv  means  of  a  light  thrown 

matometer.  ^  "  ,  ,  i         i 

upon  and  into  the  eye  through  a  lens 
by  the  action  of  a  mirror,  by  means  of  which  a  shart  com-, 
posed  of  radiating  lines  is  delineated  upon  the  rftina  of  the 
eye  to  be  examined.  The  operator  is  then  given  a  view  of 
the  figure  through  the  peep  hole  of  the  mirror,  employed. 
The  lens  employed  as  above,  when  at  its  focal  distance  from 
the  luminous  object,  transmits  parallel  rays  to  the  observed 
eye,  which,  if  emmetropic,  will  focus  the  figure  at  the 
retina  and  give  a  sharp  definition.  This  image  is  impaired 
by  any  error  of  refraction  of  the  eye  in  the  same  way  that 
vision  is  impaired  by  a  defect  of  refraction.  The  observer, 
whose  eye  is  supposed  to  be  normal,  sees  such  impairment. 
But  the  lens  is  mobile  and  may  be  so  placed  with  reference 
to  the  object  and  the  observed  eye  as  to  correct  the  impaired 
image,  or  to  correct  successively  the  impairment  of  the 
radiating  lines. 

The  position  of  the  mobile  lens  required  to  correct  and 
sharply   define  the   whole   image   or  any   mei-idian  of  it 

99 


measures  the  amount  of  the  optical  error  of  the  whole  eye, 
or  successively  of  any  two  principal  meridians  that  may 

develop  the  maximum  and  minimum 
^pri^c^pies!  ^^    error,    as    in    astigmatism.      The 

mechanism  by  which  the  mobile  lens 
is  moved  also  places  before  the  observing  eye  that  lens 
which  compensates  for  the  position  of  the  mobile  lens,  or 
both  eyes  at  once  to  the  clear  perception  of  the  figure  or  of 
one  set  of  the  radiating  lines.  The  instrument  represents 
a  high  degree  of  inventive  genius  and  a  thorough  mastery 
of  the  mathematical  principles  of  optics.  It  is  the  inven- 
tion of  John  Chambers,  formerly  of  Chambers,  Inskeep  & 
Co.,  now  consolidated  with  the  Hardy  firm.  We  under- 
stand that  Mr.  Chambers  is  now  engaged  in  the  rural  pas- 
time of  raising  chickens  for  the  market  in  a  little  Indiana 
town,  and  that  he  considered  that  the  optical  instrument 
field  had  been  sufficiently  exploited  before  retiring  from  it. 
The  stigmatometer,  because  of  its  arrangements  for  de- 
lineating an  image  on  the  retina  of  an  external  illuminated 
chart,  has,  by  the  same  means,  a  method  of  illuminating  the 
retina  itself,  or  of  delineating  the  disc,  arteries  and  veins 

and  other  details  of  the  fundus.     In 

ophthahnTscope.  ^^^^^^  ^'^^^^'  the  chart  being  omitted 
and  the  illumination  being  admitted 
into  the  eye,  it  affords  all  the  facilities  for  an  ophthalmo- 
scopic examination  by  the  direct  method.  Indeed,  the  blood 
vessels  themselves  may  be  treated  as  an  astigmatic  chart 
and  the  refraction  of  the  eye  determined  from  them,  but  as 
an  ophthalmoscope  it  offers  complete  facilities  for  the  study 
of  the  fundus  and  the  diagnosis  of  any  pathologic  condi- 
tion that  is  revealed  by  the  appearances  of  that  field.  This 
feature  of  the  instrument,  in  connection  with  its  value  in 
determining  objectively  the  refractive  condition  of  the  eye, 
adds  an  exceedingly  valuable  field  to  its  practical  useful- 
ness. 

100 


CHAPTER  "XII. 

PRACTICAL  TRAINING. 

The  technics  of  shadow  testing  are  not  difficult  and  are 
readily  understood,  but  the  practical  application  of  the 
method  requires  considerable  drill  and  training.  Essen- 
tially the  method  is  an  art  and  to  become  skilled  in  it  time 

and  patient  endeavor  are  necessary. 
woTTi.^  ^^  ^^^^  ^^^^   stated  heretofore,   one 

cannot  impose  upon  the  good  nature 
of  his  friends  to  furnish  him  with  a  subject  to  practice  on. 
He  would  doubtless  fail  upon  such  a  case  to  start  with, 
for  the  rules  would  go  a  glimmering  even  if  he  succeeded 
in  finding  the  fundus  reflex,  which  would  be  doubtful. 
With  the  facilities  afforded  by  a  schematic  eye,  which  is 
more  patient  and  submissive  than  the  best  friend  would 
be,  familiarity  with  the  motions  and  their  interpretation  is 
very  readily  acquired.  With  a  trial  case  of  test  lenses, 
most  any  sort  of  a  light  and  a  suitable  mirror  you  can  re- 
tire to  some  quiet  corner  in  an  upper  back  chamber  and 
go  to  work.  For  this  purpose  take  the  position  indicated  in 
the  illustration,  the  schematic  eye  being  elevated  to  within 
a  few  degrees  of  the  light  and  slightly  lower  than  your  own 
eye,  so  that  the  light  will  be  reflected  approximately  along 
the  visual  axis. 

The  first  thing  to  do  had  best  be  to  set  the  eye  at  the 
zero  point,  if  there  is  a  notation  on  the  inner  tube  so 
marked.  This  is  supposed  to  represent  the  position  of 
emmetropia,  although  it  cannot  be  depended  upon  to  be 

that  exact  point.    Throwing  the  light 
Thf^yef  *^  *^^  ^y^  ^^^  applying  your  eye  to 

the  peep  hole,  or  first  adjusting  the 
peep  hole  to  your  eye  and  then  casting  the  reflected  light 

103 


upon  the  eye,  endeavor  to  see  the  fundus  reflex.  You  may- 
have  a  lot  of  ''fun"  before  you  find  the  fundus.  In  the 
schematic  eye  there  are  small  images  produced  by  reflection 
from  the  surfaces  of  the  lens  that  are  to  be  disregarded  the 
same  as  similar  reflections  from  the  surfaces  of  the  eye,  and 
when  a  lens  or  lenses  are  imposed  other  reflections  may  be 
added.  The  former  are  neglected,  but  the  latter  are  often 
so  large  that  they  obscure  everjrthing  else.  They  are  easily 
avoided  by  taking  a  position  to  one  side  of  the  position  that 
produces  them.  After  finding  the  real  fundus  reflex,  the 
next  thing  to  do  is  to  tilt  the  mirror  so  as  to  cause  the  re- 
flected light  to  pass  across  the  eye  and  to  observe  the  direc- 
tion of  motion  of  the  reflex  as  compared  with  the  direction 
of  tilting  the  mirror.  In  the  case  given — that  is,  with  the 
eye  set  at  emmetropia — this  is  expected  to  be  with  the 
mirror.  Even  if  the  eye's  notation  is  considerably  out  of 
the  way  such  motion  will  be  as  expected.  A  +1.00  D.  sphere 
should  neutralize  the  eye  at  one  meter,  -\-.15  D.  at  53  in., 
and  any  lens  for  a  distance  corresponding  to  the  dioptric 
power. 

If  the  given  schematic  eye,  when  set  at  the  zero  point, 
does  not  actually,  or  practically,  neutralize  as  above,  the 
notation  is  in  some  respect  defective.  If  motion  with  the 
+1.00  sph.  continues  to  be  with  at  one  meter,  the  zero 

point  is  too  far  back  and  may  be  ad- 
Totati^^  vanced  by  a  pencil  notation.    All  the 

other  points  will  be  advanced  pro- 
portionately. But  if  motion  is  found  to  be  decidedly 
against  the  mirror  the  zero  point  is  too  far  forward,  and  all 
other  points  to  correspond.  A  verification  of  the  above 
facts  would  make  good  practice  of  itself.  A  +2.00  sph. 
should  neutralize  1.00  D.  of  hyperopia,  +3.00  D.  2  D.  of 
hyperobia,  etc.,  each  dioptric  position  requiring  1.00  D. 
more  plus  than  its  notation.    For  the  same  reason  1.00  D. 

104 


of  myopia  should  be  neutral  at  one  meter  without  a  lens, 
2.00  D.  of  myopia  require  a  — 1.00  D.  etc.,  or  at  any  posi- 
tion with  1.00  D.  less  than  the  notation.  That  is,  an  eye 
in  any  dioptric  condition  requires  1.00  D.  more  plus  to 
neutralize  it  for  40  inches  than  its  proper  correction,  and 
hence  its  dioptric  eorrectioh  is  1.00  D.  less  than  the  lens 
that  neutralizes  it  at  that  distance. 

Having  proved  up  the  notation  on  the  eye,  or  disproved 
and  corrected  it,  in  this  manner,  we  are  ready  to  proceed 
with  experimental  cases.  Set  the  eye  at  any  point,  at  first 
known  but  later  unknown,  and  proceed  to  shadow  test  it 
at  the  favored  distance  and  with  the 
^Testtnir°^  favored  mirror.     If  it  shows  by  the 

motion  that  plus  is  required,  put  be- 
fore it  +1.00,  +2.00  or  any  plus  value  tending  to  neutral- 
ize the  motion.  Do  not  be  satisfied  with  mere  neutraliza- 
tion, but  increase  the  value  of  the  kind  of  lens  required 
until  the  motion,  whatever  it  may  be  in  the  first  place,  is 
reversed.  This  is  to  make  sure  of  the  fact  that  a  sufficient 
value  is  employed.  Having  reversed  initial  motion,  proceed 
to  reduce  the  value  by  imposing  a  lens  of  the  same  kind  but 
of  a  lower  dioptric  power  in  its  place,  or  by  placing  weak 
lenses  of  the  opposite  kind  before  the  former  lens,  until  you 
are  satisfied  with  the  finding.  Having  found  the  neutraliz- 
ing lens,  make  the  required  deduction  or  addition  for  your 
distance.  The  result  should  be  the  measure  of  the  condi- 
tion, whether  hyperopic  or  myopic.  Continue  the  practice 
with  an  eye  whose  error  is  symmetrical  until,  having  set  the 
eye  at  an  unknown  point,  you  can  diagnose  the  error  in 
half  of  one  minute. 

Having  perfected  yourself  to  this  extent,  astigmatic  cases 
can  be  taken  up.  Astigmatism  is  introduced  into  a 
schematic  eye  by  placing  a  cylinder  before  it  and  testing 

107 


the  movement  of  the  reflex  and  shadow  with  such  cylinder 

in  position.     What  effects  result  will 
^yiSdeM^  depend  upon  the  set  of  eye  plus  the 

cylinder  value.  A  minus  cylinder  axis 
90  tends  to  introduce  hyperopia  into  the  180th  meridian, 
for  the  value  of  the  cylinder  is  opposite  to  the  axis  wherever 
placed.  The  cylinder  may  not  be  sufficient,  however,  to 
overcome  the  myopia  given  the  eye  in  the  setting.  In  that 
case  it  will  merely  lessen  the  myopia  in  180  to  the  extent 
of  the  cylinder  value.  In  the  same  manner  a  plus  cylinder 
at  any  axis  tends  to  overcome  hyperopia  or  to  introduce 
myopia  along  the  meridian  at  right  angles  to  the  axis  of  the 
cylinder  employed.  What  the  effect  of  either  cylinder  is  de- 
pends upon  the  set  of  the  eye  as  regards  hyperopia  or 
myopia  and  the  amount  of  either.  Such  a  cylinder  may 
Neutralize  the  meridian  at  right  angles  to  its  axis,  and  will' 
do  so  if  the  eye  is  set  at  an  amount  of  hyperopia  or  myopia 
that  the  cylinder  value  exactly  compensates  for.  If,  on 
the  other  hand,  the  eye  is  set  at  neutrality  (1.00  D.  myopia) 
the  cylinder  will  not  disturb  such  neutrality  under  its  axis, 
but  will  render  the  opposite  meridian  correspondingly 
hyperdpic  or  myopic,  or  give  motion  requiring  a  corre- 
sponding amount  of  plus  or  minus. 

The  first  test  may  show,  with  the  cylinder  on,  the  same 
motion  for  all  meridians,  either  with  or  against  the  mirror, 
but  no  sphere  will  neutralize  all  meridians  together.  If  it 
neutralizes  one  it  will  either  underneutralize  the  other — 

leave  motion  the  same — or  over-neu- 
^erm^n^^         tralize  it— give  opposite  motion.    But 

two  different  spheres  can  be  found, 
one  of  which  will  neutralize  one  of  the  meridians  and  the 
other  the  other  meridian.  The  difference  between  the 
values  of  these  two  sphericals  is  the  measure  of  the  astigma- 
tism.   If,  for  instance,  a  +2.00  sph.  is  required  to  neutral- 

108 


ize  the  vertical  meridian  and  a  +3.00  sph.  to  neutralize  the 
horizontal,  there  is  1.00  D.  of  astigmatism.  Such  dark 
room  finding  would  show  that  to  neutralize  both  meridians 
at  the  same  time  one  of  the  following  prescriptions  would 
be  required: 

(1)  +2.00  sph.  =  +1.00  cyl.  ax.  90,  or 

(2)  +3.00  sph.  =  —1.00  cyl.  ax.  180. 

The  correction  would  then  be  found  by  making  the  neces- 
sary deduction  for  the  distance  at  which  the  test  was  made. 
For  one  meter  the  deduction  of  one  diopter  would  give 
either 

(1)  +1.00  sph.  =  +1.00  cyl.  ax.  90,  or 

(2)  +2.00  sph.  —  —1.00  cyl.  ax.  180. 

In  making  the  deduction  or  addition  for  astigmatism,  either 
reduce  the  dark  room  findings  to  a  sphero-cylinder  pre- 
scription and  then  make  the  deduction  or  addition  from 
the  sphere  only,  or  put  the  findings  for  the  two  meridians 
on  the  cross,  as  in  the  above,  +2  vert.  +3  hor.,  and  make 
the  deductions  for  both,  or  +1  vert.  +2  hor.,  and  then  re- 
duce the  latter  to  the  sphero-cylinder  form.  The  cylinder 
value  is  unaltered  by  either  method. 

If  the  cylinder  used  to  introduce  astigmatism  into  a 
symmetrical  schematic  eye  is  placed  with  its  axis  oblique, 
or  in  any  direction  other  than  90  or  180,  the  principal 
meridians  of  the  astigmatism  will  be  alone  the  axis  of  the 
cylinder  and  at  right  angles  to  it.    In 
Astigmatism  ^^ch  a  case  the  obliquity  of  the  prin- 

cipal meridians  will  be  show^n  by  the 
manner  in  which  motion  adheres  to  the  principal  meridians 
when  the  mirror  is  tilted  so  as  to  make  the  light  pass  across 
the  eye  on  any  other  meridian.  As  spheres  are  imposed 
and  one  of  the  principal  meridians  approaches  the  neu- 
tralization point,  such  adherence  becomes  more  insistent, 
clearly  marking  the  position  of  the  meridians  of  greatest 

109 


and  least  dioptric  power.  The  axis  of  the  correcting  cylin- 
der in  such  case  requires  to  stand  along  one  of  the  prin- 
cipal meridians,  and  of  course  at  right  angles  to  the  other. 
In  a  schematic  eye,  since  there  is  no  accommodative  power, 
it  matters  very  little  which  meridian  is  first  neutralized, 
but  it  is  well  to  follow  the  same  plan  as  if  the  eye  had  ac- 
commodation and  neutralize  that  meridian  that  requires 
the  greater  amount  of  plus  or  the  least  amount  of  minus 
first. 

With  a  period  of  practice  upon  the  schematic  eye  all  of 

these  movements  become  thoroughly 
Subjects  familiar  and  their  meaning  clear.  You 

can  then  call  upon  some  patient  friend 
to  pose  for  you  wKile  you  get  in  touch  with  these  matters 
as  regards  a  living  organ. 


110 


RETURN    OPTOMETRY  LIBRARY 

TO— #^    490  Minor  Hall                      642-1020 

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